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157

Ore-forming processes and mineral paragenesesof the Jáchymov ore district
Rudotvorné procesy a minerální pragenze jáchymovského rudního okrsku(77 figs, 6 tabs)
PETR ONDRU1  FRANTIEK VESELOVSKÝ1  ANANDA GABAOVÁ 1  MILAN DRÁBEK1 
PETR DOBE1  KAREL MALÝ1  JAN HLOUEK2  JIØÍ SEJKORA3
Czech Geological Survey, Klárov 3, CZ-118 21, Prague 1
U Roháèových kasáren 24, CZ-100 00, Prague 10
3
National Museum, Václavské námìstí 68, CZ-115 79, Prague 1

1

2

The contribution provides characteristics of the so-called five-element AgNiCoBi±U formation, points to differences in sources of the
U formation and the AgNiCoBiAs formation, and presents a model of element fractionation in abyssal hydrothermal sources according
to physical-chemical properties of the compounds. The role of arsine (AsH3) in the mobility of the triad NiCoFe is discussed. A new
scheme of mineralization stages is proposed in relation to three sources, separated in space and time: 1. greisenization of granite: SnW
sulpharsenide stage. 2. U formation: carbonateuraninite stage. 3. AgNiCoBiAs formation: arsenide, arsenicsulphide, and sulphidestages. A new geochemical and thermodynamic interpretation of hydrothermal processes of individual mineralization stages is presented,
including a detailed discussion of thermodynamic conditions of UVI mobility and uraninite precipitation. Redox potentials of hydrothermal
environment after the carbonateuraninite stage including crystallization of silver and bismuth dendrites, precipitation of NiCoFe
polyarsenides and electrochemical dissolution of Ag and Bi dendrites are also discussed. Mechanisms of the precipitation of base metalsulphides and sulpharsenides are considered in relation to redox conditions and sulphur fugacity in the environment during individual stagesof mineralization. Phase relations are used for the deduction of the conditions of formation of selected ore minerals. Diagrammatic interpretation of chemical analyses and substitution trends of selected minerals or mineral groups, and a paragenetic relation of primary and secondary minerals are used in the interpretation. A discussion of the dependence of weathering processes on mining activity is included. Data on
fluid inclusions and stable isotopes are used for the interpretation of ore-forming processes.
Key words: AgNiCoBi±U formation, mineralization stages, redox potentials, uraninite precipitation, Ag, Bi precipitation andelectrodissolution, NiCoFe polyarsenides, fluid inclusions, stable isotopes, Jáchymov.

ELEMENT SOURCES AND FRACTIONATIONS
Mineralization sources
V. Zoubek suggests Proterozoic sediments containing local accumulations of uranium minerals as a source of U;
the uranium is assumed to be released during the recrystallization of rocks (temprok 2003). Other hypothesis(Baumann et al. 2000) quote accessory minerals of granites as a uranium source; the release of uranium is associated with processes of metamorphosis of the host rocks
in this case. Recent studies date the U-mineralisation aslate-Variscan (i.e., Upper Paleozoic)  300240 Ma (Baumann et al. 2000), that means younger than greisens.
A connection of five-element mineralization to granites is assumed to be structural only instead of genetical(Baumann et al. 2000). Hence, the metals required were
not released from granites but they came from unknown
deep-seated sources and deposited in weakened structuresthat served also as feeder channels for granite intrusions.
The feeder pathways for ore-forming solutions coincidewith deep-seated faults of Gera-Jáchymov fault zone. The
ore mineralisation resulted from post-Variscan tectonomagmatic re-activation which had a form of brittle tectonics. The mineralization is dated as post-Variscan (Mesozoic) with age 240100 Ma.

Abyssal hydrothermal sources  a model for
separation of elements
At least three independent thermal sources instrumental
in the transport of ore-forming elements into the fractured
crystalline complex and their accumulation are considered for the Jáchymov ore district.
1. The oldest thermal source is related to the intrusionof granite (Karlovy Vary pluton), underlying the complex of metamorphic rocks. Greisenization related toyounger granite resulted in the enrichment of the apical parts of granite in Sn, W, As, S, Fe, Cu, (Mo, Cd,
In), a.o.
2. The second source in the time sequence is responsible for the transportation of uranium in the formof uranyl-carbonate complexes into fractures inolder granite and in the metamorphic complex. Information on the source of uranium is not available.
3. The youngest thermal source is responsible for the introduction of AgNiCoBiAs into pre-existing andpartly mineralized open fractures. Hydrothermal fluids penetrated along fault zones from a crustal domain
below the granite massif. The ultimate source of theseelements is again unknown.

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Fractionation of elements of the AgNiCoBiAs
formation
AgNiCoBiAs formation
The character of the individual periods of mineralization
is interpreted as a consequence of the fractionation of Ag,
Bi, Ni, Co, Fe and As compounds in a 3D domain of anevolving hydrothermal system. The process includes repeated crystallization and selective separation of elements
controlled by differences in physical-chemical propertiesof their compounds with non-metallic elements: S, As,
Cl, F, O (Fig. 1).
Silver and bismuth are the first fractionated elements
due to the volatility of Ag and Bi chloro-complexes inhydrothermal environment under elevated temperatures.
The subsequent fraction contains Ni, Co, Fe bound involatile complexes of the MeX4 or MeX6 type (where
X = Cl, OH, H2O and AsH3). With regard to new data[578], separation of these elements by the formation of pentacarbonyl-like phases with ligand AsH3, or substitution of
AsH3 for CO (e.g., phase as P analogue of Fe(CO)4(AsH3)
with dissociation energy of ca. 40 kcal/mol [578]) is possible. Other possible ligands are: N2, CCH2, CH2, H2,
NH3, PH3 and many others. Evidence of the occurrenceof Ni(CO)4 in fermentation gases from a municipal sewage treatment plant was presented in [428].
This fraction is followed by a fraction containing only
As(S) without significant metal concentration. In its final stages, these products were succeeded by the last frac-tion containing base metals (Pb, Cu, Zn, Fe) and S; arsenic content is insignificant.
The five-element association documented in the Jáchymov ore deposit with its succession of minerals corresponds
to the outlined mechanism of element fractionation. The
fact that certain minerals occur in several generations andin part in various oxidation states is interpreted as due tooscillating redox conditions and rejuvenations. For example, silver was primarily deposited as native metal, but was
re-mobilized and deposited at other sites, under changed
conditions, in the form of sulphides or sulphosalts.
PARAGENETIC RELATIONS OF THE ORE VEINSYSTEM
Mineralization stages
Tectonic movements separated individual mineralization
stages responsible for the deposition of the mineral contents of ore veins [374]. Deposition of vein minerals took
place in several chemically specialized stages of mineralization. This development was accompanied by repeated opening of fractures and by fracturing of older veinminerals. Tectonization opened conduits for mineralizing
solutions propagating along NWSE-striking fault zones
and lower-order faults. Tectonic effects accompanying the
Tertiary volcanism included mainly movements on EWstriking faults and the emplacement of basaltic breccia
dykes. In places where ore veins were deformed by fault
reactivation, their ore minerals were crushed and exposed

Fig. 1. Estimated trends of variation with time in activities of major elements, Eh and temperature during the main mineralization
stages.

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to enhanced groundwater flow, resulting in a stronger secondary alteration. The existing mineral filling of ore veins
is not significantly complicated by tectonics post-dating
the mineralization [374].
The complicated mineralization succession can be
partly reconstructed only on the basis of the preserved
information and material. The value of this information
is weakened by the profound effects of the younger stagesof mineralization. Only the middle and lower parts of theveins have been preserved, while the upper parts were
removed by erosion.
The above complications affected the interpretation ofmineralization stages and of sources of ore-bearing solutions, presented by various studies of the problem:[386], [431], [435], [436], [437], [438]. The time succession of mineralization stages proposed by Mròa and Pavlù
[351], [431], is presented here, modified with respect tothe data of the present study.

159

Fig. 3. J109P/2. 1  covellite, 2  stannite, 3  bornite, 4  chalcopyrite. Klement shaft. BSE image. Magnification 1150×.

Detailed characteristics of mineralization stages
SnW sulpharsenide stage
This stage is composed of several independent periods,
which belong probably to high-temperature pneumatolytic and high-temperature hydrothermal mineralization
stage known from Cínovec or from greisens of the Horní
SlavkovKrásnoÈistá area, also related to young granites.
The authors of this study are convinced that this mineralization stage, or at least its younger period, is identical withthe older sulphide stage defined by previous authors.
This stage of mineralization is probably the oldest one,with temperatures of ~300 °C (see Fluid inclusion study).
The mineral paragenesis of SnW sulpharsenide mineralization is rare in the Jáchymov ore district. It was deposited on EW structures, but the local characteristics ofdeposition are rather variable. It has been recorded inFig. 2. J061P. Stannite replacing euhedral cassiterite is rimmed by chalcopyrite. Aggregate of intimate intergrowths of aikinite and matildite
are at the left margin of stannite. Corroded arsenopyrite grains are enclosed in chalcopyrite. 1  stannite, 2  chalcopyrite, 3  arsenopyrite,
4  assiterite, 5  aikinite, 6  matildite, 7  gangue. Adit No. 20.
Reflected light, single polarizer. Magnification 65×.

Fig. 4. J109P/3. 1  stannite, 2  chalcopyrite. Klement shaft.
BSE image. Magnification 160×.

the Trojická vein, Svornost Shaft, Daniel level. In the
Rovnost I mining field, minerals of this paragenesis are cut
and displaced by about 0.5 m by veins of the five-element
mineralization [431]. The Giftkies and No. 20 adits also
encountered SnW sulpharsenide mineralization.
Minerals of this stage are not directly tied to the fracture system but represent accumulations in greisenizedrocks. Rare and weak manifestations of this stage havebeen observed in some veins striking mainly NESW andEW, e.g., Trojická vein. Principal minerals of this stage
include quartz, pyrite, arsenopyrite, tourmaline, phlogopite, tungstenian rutile, cassiterite and molybdenite andyounger sulphides: chalcopyrite, bornite, tennantite, argentotennantite, freibergite, aikinite, matildite, dark
sphalerite, galena, stannite, kësterite, mawsonite, andgersdorffite (Figs 24). These minerals occur in quartz
veins or as impregnations in strongly altered greisens, invarious combinations or isolated.
Interesting is the accessory occurrence of microscopicroquesite enclosed in mawsonite and stannite, which indicate increased contents of In in the solutions. This points
to a relationship of the Jáchymov SnW sulpharsenide
stage with the closely similar paragenesis at Cínovec [448].

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Ore-free quartz stage
This stage is characterized by various varieties of quartz,
carbonates and rare fluorite. The oldest mineral is quartz,
including chalcedony-like, ferruginous, amethystine
quartz and quartz of pectinate texture. The grey chalcedony-like quartz carries inclusions of microscopic pyriteand clay minerals. Ferruginous quartz is brown red dueto the pigmentation by Fe oxides and is locally impregnated by fine-grained chalcopyrite. Both types of quartz
constitute characteristic vein filling of lower parts of theNS veins. Pectinate quartz also widespread is either colourless or pigmented occasionally at crystal tips. It is
deposited on fracture walls, fragments of wall-rock or on
the breccia of the two above mentioned types of quartz.
Significantly less abundant are light green fluorite and(Fe,Ca)-carbonates, affected by replacement. In the interpretation by Dymkov [436], this stage would correspond to quartzfluorite paragenesis, positioned at the beginning of the carbonateuraninite stage.
Carbonateuraninite stage
Products of this stage are present in variable quantities
in a large number of veins striking NWSE and NS.
Hydrothermal solutions of this period were moderately
tempered near 200 °C, mineral paragenesis crystallized
under moderately oxidizing, neutral, and weakly alkalineconditions, which varied considerably. Sulphur fugacity
was low, locally increased SiO2 activity resulted in thecoffinitization of uraninite.
The main mineral is colloform, botryoidal uraninite
accompanied by pink dolomitic carbonate, sporadic dark
violet fluorite, grey to black quartz and pyrite. Radioactivity around uraninite aggregates turned fluorite tonearly black in colour. Carbonate crystals near uraninite
contain thin layers rich in Fe oxides, indicating growthzones.
Uranium deposited as uraninite was repeatedly remobilized and participates in mineral paragenesis of nearly
all mineralization stages.

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absent and uraninite was deposited directly in crushed
and hydrothermally altered wall rock.
Uraninite mineralization often developed in significant
concentrations, with uraninite prevailing over mineralsof younger stages of mineralization. The original extent
of uranium mineralization was larger than encounteredduring mining, since some parts of uraninite were replaced by hydrothermal solutions of the arsenide stage.
Uraninite at Jáchymov has relatively high contents of
REE. In the course of younger evolution of veins, several events of mobilization and regeneration of uraninite
took place as indicated by the range of radiometric age
determinations (see Geology and hydrothermal vein system of the Jáchymov ore district).
Other ore minerals, mainly arsenides, sulphides (dominantly chalcopyrite and galena), localized mainly infractures of uraninite aggregates or at their margins, areyounger than uraninite and belong to the paragenesis oflater mineralization stages. Dymkov [436] considered thatcalcite, later subject to dolomitization, originally accompanied uraninite.
Arsenide stage
By volume, mineralization of the arsenide stage represents the major part of the total mineralization of the AgNi-Co-Bi-As formation.
Successive relations among minerals of this stage are
discussed by Keil [440], Dymkov [436], Zückert [423],
Mròa and Pavlù [351], [431] and others.
This stage is represented in a lower number of veinsthan uranium mineralization but is more varied and themost complicated in the district. It is characterized by low
fugacity of sulphur and high fugacity of arsine. Hydrothermal mineralizing solutions were medium- to low-temperature, up to 200250 °C, mineral assemblages crystallized
in a strongly reducing, neutral or weakly acid environment.
The main gangue mineral is semitransparent or strongly pigmented quartz with frequent fine and dark grey ore
impregnation. Arsenide ores are occasionally localized in

Detailed characteristics of uranium mineralization
Uranium mineralization cemented fragments of the older quartz gangue, penetrated it in thin veinlets and, in other instances, formed with associated carbonates the solethin veinlets filling. Uraninite usually formed monomineralic massive botryoidal aggregates only several cm
thick, but extending for meters along the vein less frequently as short veins showing a symmetric structure. The
associated gangue was dolomite or dolomitic carbonate,coloured in proximity of uraninite to creamy, buff pinkor red. Dolomite is usually coarsely crystalline, coloured
by Fe oxides.
Less common gangue minerals are fluorite, dark violet to black near uraninite. Uraninite was locally accompanied by pyrite, especially where dolomitic gangue wasFig. 5. Alternating layers of nickeline (ni) and rammelsbergite (ra).
Svornost shaft, Geschieber vein. Magnification 83×. [524].

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dolomitic carbonates. However, it is uncertain whether
these carbonates belong to the arsenide stage or represent re-mobilized material from older vein filling.
In some cases, ore accumulations of diarsenides andtriarsenides show features of a rapid growth under conditions of oscillating Ni/Co/Fe ratio. The mineralizing
solutions were probably rather concentrated. A change in
p-T conditions probably triggered the deposition of gellike amorphous precipitates. A subsequent crystallizationof such materials resulted in the formation of aggregateswith contraction fractures filled by quartz or younger
generations of ore minerals.
Crystals, if developed, are nearly always zoned (nickel-skutterudite). Such changes are generally documented by repeated alternation of nickeline and rammelsbergite (Fig. 5) or by polymineral arsenide rim around silver
or bismuth.
Primary silver (bismuth) crystals served as crystallization (nucleation) sites for arsenides and diarsenides.
The main part of ore minerals consists of silver, bismuth, nickeline, NiCoFe diarsenides (safflorite, rammelsbergite, löllingite) and triarsenides (skutterudite,nickel-skutterudite). Minor aggregates of uraninite, remobilized from the uranium stage, represent a small heterogeneous component in the arsenide assemblage.
Arsenides are associated with alteration products ofboth native metals  regenerated bismuth, bismuthinite,argentite, sternbergite, argentopyrite, silver, a.o., less frequently alteration products of arsenides such as gersdorffite, glaukodot, alloclasite, pararammelsbergite, krutovite, a.o.
Several separate parageneses can be distinguished in
the arsenide stage. Depending on the presence or absenceof one of the two native metals, Ag and Bi, Zückert [423]
introduced the following division:
Silver paragenesis
 silver + Ni arsenides  dendritic crystals of silver overgrown by nickeline and rammelsbergite. Exsolved
gersdorffite has been occasionally observed.
 silver + nickel-skutterudite  crystals of native metal
are surrounded by nickel-skutterudite, followed byyounger nickeline and rammelsbergite
 silver + rammelsbergite + safflorite  larger-size silver dendrites are overgrown by a broader rim of rammelsbergite, coated by a thin layer of safflorite
Bismuth paragenesis
 bismuth + safflorite  skeletal bismuth crystals surrounded by safflorite
 bismuth + skutterudite  bismuth shapes depend on
mutual ratio of both components in solution
 bismuth + Ni arsenides
 bismuth + safflorite + rammelsbergite  skeletal bismuth
crystals are surrounded by thin rammelsbergite with abroader rim of safflorite or saffloritelöllingite161 bismuth + safflorite + skutterudite  unlike the association with safflorite, simple dendrites of bismuth aresurrounded by skutterudite with a thin rim of safflorite
 bismuth + löllingite  löllingite forms thin rims on bismuth
Transitional paragenesis
 native Bi, rammelsbergite, nickeline
Arsenide paragenesis free of native metals
 nickel-skutterudite, skutterudite, nickeline, löllingite
Minerals of the bismuth paragenesis are more common
than minerals of the silver paragenesis [383]. Both theparageneses often occur in the same vein or fracture filling but are sharply separated and form independent lenses. Common occurrence of bismuth and silver is exceptional in the form of irregular bismuth aggregates aroundsilver dendrite. Krause [424] described vertical zoning in
the Jáchymov veins, with predominance of silver in upper
levels, gradually replaced by bismuth towards deeper
levels.
The period of mineralization was probably of weaktectonic activity, and the flow velocity of the mineralizing solutions must have been very low as indicated by
the presence of delicate and brittle aggregates including
dendrites, skeletal or quill-like crystals.
Macroscopic description of mineral parageneses
A common feature of the parageneses is the presence ofwhite isomorphous diarsenides and triarsenides constituting two series:
 anisotropic series: rammelsbergite NiAs2  safflorite
CoAs2  löllingite FeAs2
 isotropic series: nickel-skutterudite NiAs3  skutterudite CoAs3
NiCoFe arsenides occur in the main paragenesis (i.e.,silver and bismuth) as typical rims surrounding silver andbismuth (predominance of diarsenides), as well as independent grains and aggregates of variable shape. Most
common are euhedral crystals  cubes of triarsenides,
orthorhombic crystals of diarsenides, often in triplets having the shape of six-pointed stars.
The alternation of sharply defined layers is typical ofpolyarsenides (Fig. 5). The layers are often separated bya thin layer of remobilized uraninite of a younger generation.
The precipitation of polyarsenides took place in the succession:
arsenides ⇒ diarsenides ⇒ triarsenides,
and the oscillation in gradually increasing redox potential resulted in a partial repetition of the succession.

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Silver paragenesis
This paragenesis includes ore minerals in dendrites forming mainly perimorphs after silver, which was largely dissolved during later hydrothermal processes. Resulting
vugs and cavities were partly or completely filled withyounger minerals, mainly ore-stage quartz, silver minerals (dominantly argentite) and sulphides. The main minerals of this paragenesis with Ni prevailing above Co arenickel-skutterudite, rammelsbergite, nickeline, rare safflorite, or löllingite.
Wire silver observed in occasional cavities does notbelong to this paragenesis, since it was deposited later.
Rather common is the alternation of thin botryoidal zones
of nickeline with thicker layers of diarsenides and triarsenides. The precipitation of nickeline mainly occurred at
an earlier time of the arsenide stage, with nickeline directly surrounding silver dendrites.
The analysis of minerals filling perimorphs of arsenides after silver indicates that silver was dissolved bysolutions of the sulphide stage. The replacement of silver by diarsenides was not confirmed.
Dendrites with arsenide perimorphs after silver often
reach the size of 5 cm and are hollow. The perimorphs
were classified on morphological basis by Mròa andPavlù [383] as: dendrites with good crystal shape definition with
branching corresponding to edges of cube (Fig. 6),
 dendrites well developed in one direction (Fig. 7),
 dendrites containing cavities which are imperfectly
crystallographically defined (Fig. 8).

Fig. 6. Perimorphs of diarsenides of the 3rd generation (rammelsbergite, safflorite) after silver, quartz is black. Eliá mine. Natural size. Mròaand Pavlù [383].

Bismuth paragenesis
Skeletal and quill-like bismuth crystals (Figs 910), often twinned, up to 2 cm long, are characteristic. Subhedral and irregular grains also occur. This paragenesis includes veinlets of younger bismuth, formed by theregeneration of primary bismuth and its deposition in
fractures of gangue minerals. Owing to limited mobilityof BiCl4 complexes (liable to hydrolysis to BiOCl inslightly acid environment), the transport took place overthe distance of several centimetres only.
Bismuth is common in paragenesis with arsenides
containing increased Co and As, mainly skutterudite,safflorite, less frequently löllingite, rammelsbergite andbismuthinite. Such replacement is locally so intensivethat perimorphs form. Galena and sphalerite are rimmed
by a thin layer of diarsenides in the proximity of skeletal bismuth. Nickeline, typical of silver paragenesis, israre in bismuth paragenesis. In a similar way as withsilver dendrites, NiCoFe diarsenides form rims
around skeletal bismuth crystals or fill cavities in bismuth. Unlike silver, bismuth may form lenses free ofother ore minerals.
Analyses of ore lenses of bismuth paragenesis [351]show Ni : Co ratio up to 2 : 1, and cases of Co predominance occur. Notable is an increased As content in bis-

Fig. 7. Silver paragenesis, nickeline (stippled) is closely rimmed bywhite rammelsbergite of 2nd generation, in the upper part dendrites ofdiarsenides of the 3 rd generation, quartz is black. Eduard shaft. Natural size. Mròa and Pavlù [383].

Fig. 8. Silver paragenesis, thin rims of diarsenides coating cavities after
silver, quartz is black. Eliá mine. Natural size. Mròa and Pavlù [383].

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163

Fig. 9. Arsenide mineralization with bismuth. Svornost shaft, Geschieber vein. BSE image. Magnification 15×. [524].

Fig. 11. J016P. Gersdorffite enclosed in radiating aggregates of nickeline. Rims of nickeline are formed by nickel-skutterudite with zonal
texture and replaced by marcasite. 1  nickeline, 2  gersdorffite,
3  nickel-skutterudite, 4  marcasite. Bratrství shaft. Reflected light,
single polarizer. Magnification 160×.

Fig. 10. Quill-like bismuth crystals rimmed by safflorite. Svornost
shaft, Viktorie vein. BSE image. Magnification 65×. [524].

Fig. 12. J050P. Native bismuth on nickel-skutterudite is replaced bybismuthinite. Oval grains of uraninite at nickel-skutterudite-rammelsbergite contact. 1  bismuth, 2  bismuthinite, 3  nickel-skutterudite,4  rammelsbergite, 5  uraninite. Rovnost. Reflected light, single polarizer. Magnification 130 ×.

muth ores with Bi/As ratio of 1 : 2 to 1 : 2.8. In the silver paragenesis the Ag/As ratio is 1 : 1.5 to 1 : 2.3.
Mineral succession of the arsenide stage
The scheme of the precipitation succession of individual
minerals of the arsenide stage is complicated by local
physical and chemical conditions in a given part of thevein. The crystallization sequence is complicated by thesuperposition of a new generation of certain mineral andby the absence of some minerals in parts of the vein.
The compilation and evaluation of analytical and published data [351], [431] provided a basis for the schemeof mineralization succession in silver paragenesis of thearsenide stage:
ferruginous quartz ⇒ dendritic silver ⇒ spherulites ofregenerated uraninite ⇒, nickeline, nickel-skutterudite(larger aggregates) ⇒ rammelsbergite (less frequently
safflorite), nickeline ⇒ rammelsbergite (rimming nickeline) ⇒ skutterudite (mainly isolated crystals) ⇒ ore
quartz, rare carbonates.

Fig. 13. J056P. Corroded cubic crystals of skutterudite grown on
safflorite, which rims original rammelsbergite radial aggregates withcentres of coffinitized uraninite. 1  skutterudite, 2  safflorite,
3  rammelsbergite, 4  coffinitizated uraninite, 5  violarite,
6  vein stuff. Svornost shaft, 5 th level, Prokop(Co) vein. Reflected
light, single polarizer. Magnification 130 × .

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Fig. 14. J056P. Radial aggregates of acicular rammelsbergite enclose
botryoidal uraninite, which is slightly coffinitized. Safflorite crystals
grew on rammelsbergite. 1  safflorite, 2  rammelsbergite, 3  uraninite and coffinite, 4  gangue. Svornost shaft, 5th level, Prokop(Co)vein. Reflected light, single polarizer. Magnification 260×.

Fig. 16. MP290C/H-5. 1  löllingite, 2  gersdorffite, 3  nickel-skutterudite. Barbora shaft, 5 th level, vein No. 32. BSE image. Magnification 20× .

aFig. 17. MP511C/B-2. 1  nickeline-(NAP), 2  uraninite, 3  rammelsbergite, 4  annabergite, 5  dolomite. Rovnost II shaft, 6th level,vein No. 16. BSE image. Magnification 60×.

b
Fig. 15. J117P. The oldest nickel-skutterudite is corroded and dissolved
and it is rimmed by rammelsbergite. The arsenides are in turn enclosed
in zonal nickeline, which alternates with another generation oframmelsbergite. Border between monoarsenide and diarsenide is sharp.
Chalcopyrite and proustite form minor inclusions near nickel-skutterudite. 1  nickeline, 2  rammelsbergite, 3  nickel-skutterudite,4  chalcopyrite, nickelie 5  carbonate. Eliá mine, 2A vein. Reflected
light. Magnification 100×.
a) single polarizer b) crossed polarizers

Fig. 18. J115P/B-1. 1  arsenolite, 2  matildite, 3  nickel-skutterudite.
Rovnost II shaft, 6th level, vein No. 16. BSE image. Magnification 800×.

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165

Fig. 19. J117P/A-8. 1  galena, 2  nickeline a rammelsbergite. Eliámine, 2A vein. BSE image. Magnification 16×.

Fig. 22. MP215. 1  bismuth, 2  uraninite, 3  rammelsbergite,
4  perimorphs after silver. Tomá shaft. BSE image. Magnification 100×.

Fig. 20. J120P/B-1. 1  bismuth, 2  nickeline, 3  rammelsbergite,
4  S-rammelsbergite. Barbora shaft, 5th level, vein No. 32. BSE image.
Magnification 130×.

Fig. 23. MP215/1. 1  bismuth, 2  uraninite, 3  rammelsbergite,
4  nickel-skutterudite. Tomá shaft. BSE image. Magnification 32×.

Fig. 21. MP41/D-2. 1  rammelsbergite, 2  Fe-gersdorffite, 3  nickel-skutterudite. Svornost shaft, 10th level, Geschieber vein, BSE image.
Magnification 750×.

The succession of the bismuth paragenesis is similar:
ferruginous quartz ⇒ bismuth ⇒ spherulites of regenerated uraninite ⇒ skutterudite (larger aggregates) ⇒safflorite (or rare rammelsbergite) ⇒ löllingite, skutterudite (mostly colloform structures) ⇒ [galena, sphalerite] ⇒ ore quartz, rare carbonates.
Later or concurrently with the above-mentioned stages of arsenide mineralization, a number of other processes
took place:
Native metals deposited as dendrites were partly orcompletely dissolved, nickeline was decomposed, secondary Bi and Ag minerals crystallized, precipitation of carbonates, quartz and copper minerals in vugs after leachednative metals or among aggregates of arsenides took place.
The basic scheme is complicated by rhythmic crystallization of arsenides (mainly rammelsbergite, gersdorffite
and nickeline; Fig. 11), by several generations of regen-

166

erated uraninite, by the presence of younger dendritic silver
deposited in sheaf-like aggregates on arsenides, the existenceof several generations of bismuth, a.o. Other typical and characteristic minerals of this period are: bismuthinite, anhedral bismuth, argentite (Figs 1123).
Arsenicsulphide stage
Besides arsenic, sulphur and to some extent also antimony participated in the hydrothermal process of this mineralization stage. Typical is an increase in redox potentialof ore-bearing solutions, neutral or weakly acid conditions
and crystallization temperature in the range of 180200 °C
in early stages and ca. 60 °C in its closing stage.
This paragenesis occurs in numerous veins, particularly in the central and southwestern part of the district. The
paragenesis occurs in a significant quantity and a widevertical range in the Geschieber vein. Ores of this mineralization stage form either independent accumulations
or associate with minerals of the arsenide stage, or withthe youngest base metal sulphide mineralization stage.
The most common ore mineral is arsenic, accompanied
by silver mineralization, sometimes with realgar, e.g., in
the upper part of the Geschiber vein, and with löllingite.
The main gangue mineral is dolomitic carbonate.
One of the oldest minerals of this stage is dendritic
silver enclosed in lenses of arsenic. Leaching of silver
or its substitution by proustite took place during thisstage. Silver also occurs in globular aggregates directly in massive arsenic.
Silver paragenesis is represented by the following minerals: Sb-proustite, As-pyrargyrite, stephanite (importantamounts in lower parts of the Geschiber vein), less frequent argentite, pyrargyrite (without proustite), polybasite, arsenpolybasite, xanthoconite and small crystalline
aggregates or isolated crystals of sternbergite and argentopyrite. Silver is not represented to such degree as in thearsenide stage. Silver minerals of this paragenesis are
dominantly deposited on arsenic or fill vugs in arsenic.
The single arsenide observed in silver paragenesis islöllingite, which rims arsenic aggregates in the form ofsmall crystals.
In some important veins (Geschieber, Hildebrand), thearsenicsulphide mineralization stage shows increased Sb
contents which resulted in the following association of
Sb minerals: stibnite, pyrargyrite, antimony, dyscrasite,
miargyrite, pyrargyrite, robinsonite, stibarsen, and AsSb
and BiSb inter-metallic phases. Mròa and Pavlù [351]
noted vertical zoning in Sb abundance in the Geschieber
vein. It is demonstrated by the occurrence of Ag-sulphantimonates in lower levels, while in upper levels the content of Sb is lower, and Sb is contained mainly as equantadmixture in proustite. Pyrite is also present in the above
association of Sb minerals.
Sb facies of mineralization
Minerals containing Sb, or with prevalence of Sb above
As, occur in the whole ore district, but are less numerous than minerals with prevailing As.

Journal of the Czech Geological Society 48/34(2003)

Sb is mainly contained in minerals of the proustite
pyrargyrite and tennantitetetrahedrite series. It is impossible to identify crystallization sequence from the intergrowth of these minerals. In a similar way, relative
succession cannot be determined from tennantites enriched in Sb. The presence of tetrahedrite filling fracture
in quartz containing tennantite grains shows that tetrahedrite in the sample from Giftkies adit is younger thantennantite.
Sb content in gersdorffites is increased in late crystallization stages. In a similar way, Sb-bismuthinite crystallized in later stages.
The Hildebrand vein in the Svornost mine is known
from historical records through the occurrence of
Sb-bearing minerals. However, only a small number ofprecisely localized samples have been preserved fromthis fairly small vein, which occasionally yielded somegood ores. The samples show that at least some partsof the vein reached notable enrichment in Sb, whichenabled the origin of an anomalous mineral paragenesis for the Jáchymov deposit. Antimony was locallyformed as inclusion in miargyrite, which is in equilibrium with Sb. Exceptionally, pyrargyrite occurs in thisassemblage.
Arsenic also occurs in the paragenesis with Sb minerals. Its younger parts contain increased Sb contents ofmax. 7 wt. % and inclusions of dyscrasite. This situation
indicates approximately the time interval in which the Asdominated evolution was terminated and succeeded by aphase with increased Sb concentration. Tectonic effects
coinciding with the onset of this change are indicated byveinlets of younger arsenic filling thin fractures incoarse-grained dolomite and calcite in the proximity ofolder arsenic.
Obviously one of the youngest (in the stage) minerals
is radiating stibnite resting on a band of botryoidal pyrite and enclosed in whitish calcite. Terminal parts ofstibnite crystals are partly replaced by spheres of lightyellow sphalerite. This indicates that Sb largely remainedin solutions during the crystallization of ore minerals andincreased in concentration during later stages.
Sulphide stage
Minerals of this stage are distributed fairly extensively, but the accumulations are minor, developed independently of older mineralization or deposited on older
ores. Minerals of this stage fill contraction fracturesin uraninite (carbonateuraninite stage) and marginal, fractured, or leached volumes in ores of arsenide
stage.
The main minerals are galena with a low Ag content,
sphalerite (light with low Fe, In, Cd), chalcopyrite, pyrite, marcasite and sometimes arsenopyrite. Isolated finds
include bornite with exsolved chalcopyrite, boulangerite and tennantite, accompanied by accessory chalcopyrite, sphalerite and galena [351] (Figs 2432).
Predominant gangue mineral is calcite, containing impregnations of sulphides or alternating with sulphides inbanded structures.

Journal of the Czech Geological Society 48/34(2003)

167

Fig. 24. J059P. Concentric aggregates of chalcopyrite alternate withtennantite. Rammelsbergite preserved in relics. As-chalcopyrite forms
pink brown layers. 1  chalcopyrite, 2  tennantite, 3  As-chalcopyrite,
4  rammelsbergite, Svornosr shaft, 5th level, Prokop vein. Reflected light,
single polarizer. Magnification 130×.

Fig. 27. J183P/4. 1  sphalerite, 2  pyrite, 3  smythite and pyrite,
4  Fe-dolomite. Svornost shaft, 2nd level, Hildebrand vein. BSE
image. Magnification 40×.

Fig. 25. Uraninite spherule enclosing rock fragment with relics ofsilver and fissure filled with chalcopyrite and bornite altered into
covellite. Tennantite and silver in proximity. 1  silver, 2  chalcopyrite, 3  bornite, 4  tennantite, 5  covellite. Svornost shaft, Daniellevel, Trojická or Prokop vein. Reflected light, single polarizer.
Magnification 260×.

Fig. 28. MP41/D-4. 1  rammelsbergite, 2  matildite and mimetite.
Svornost shaft, 10th level, Geschieber vein. BSE image. Magnification
600×.

Fig. 26. J117P. 1  rammelsbergite, 2  chalcopyrite, 3  nickel-skutterudite, 4  argentite. Eliá mine, 2A vein. Reflected light, single polarizer. Magnification 100× .

Fig. 29. MP41/D-5. 1  matildite and Bi-arsenate. Svornost shaft, 10th
level, Geschieber vein. BSE image. Magnification 500×.

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Journal of the Czech Geological Society 48/34(2003)

Post-ore stage

Fig. 30. MP72/A. 1  sphalerite, 2  galena, 3  nickelskutterudite.
Eliá mine, 2A vein. BSE image. Magnification 80×.

This stage was divided in the literature [351], [423] into
several periods, such as Mn-rich calcite, fluoritebarite,
or opalquartz, with variable manifestations in various
parts of the deposit. In general, the post-ore stage is indistinct in the Jáchymov ore district.
Dominant mineral of this stage is Mn-rich calcite, filling offshoots branching from the major NWSE-striking
fault zones. Minerals deposited in this stage show limited effects of younger tectonic deformation. The mineralization of the fluoritebarite stage is poorly defined;some fluorite occurrences belong to older mineralization
stages, and barite is too rare. For these reasons, relationsof post-ore stage to the general succession of the deposit
are not sufficiently clarified.
Gangue minerals of individual mineralization stages

Fig. 31. MP290C/H. 1  sphalerite, 2  galena, 3  wittichenite,
4  bismuthinite. Barbora shaft, 5th level, vein No. 32. BSE image. Magnification 1600×.

Fig. 32. MP386/1. 1  wittichenite, 2  bornite. Plavno, Vladimír mine.
BSE image. Magnification 200×.

Important gangue minerals include carbonates mostly ofthe dolomite  ankerite series, quartz in several generations, minor fluorite, and exceptionally barite and siderite. Dolomite and Fe-rich dolomite were observed in majority of veins, but pure calcite was fairly rare, its separate
veinlets formed in later stages of mineralization, partly
by the recrystallization of dolomitic carbonates.
Although carbonates represent the main gangue minerals, the total volumes of carbonates are quite low. Thissituation is responsible for strongly acidic conditions at
the deposit. The relatively low carbonate buffering capacity could not neutralize the aggressive arsenate- andsulphate-rich solutions. The solutions circulated throughlarge volumes of the deposit and produced a wide rangeof secondary minerals.
Quartz of the oldest generation was described as grey,hornstone quartz. Drusy quartz of the second generation
is widespread in the district. It occurs as early depositionin open fractures, later filled by uranium and complex
ores. Its deposition marked the beginning of ore mineralization. Quartz of the third generation is white, grey andbrown, sometimes rather fine-grained. This generation
occurs mainly in the parts of veins with carbonates.
Carbonates filled many vein offshoots, especially insections where the main vein was filled by mylonite.
Carbonate formation is represented by dolomite, Mg-rich
calcite or Fe-rich dolomite and ankerite. Yellow and pinkcolours prevail but dispersed hematite turns carbonates tobrown colour. In the proximity of uranium accumulations
the carbonates are typically brick red. Calcite crystallized
after the deposition of the dolomiteankerite carbonatesand occurs only in small amounts. Its typical colour iswhite. The least common carbonate is siderite.
Fluorite is relatively rare. It was deposited on drusy
quartz, before the crystallization of carbonates, and either before or after the deposition of uraninite.

Journal of the Czech Geological Society 48/34(2003)

GEOCHEMICAL AND THERMODYNAMIC
INTERPRETATION OF HYDROTHERMAL
PROCESSES OF INDIVIDUALMINERALIZATION STAGES
Carbonateuraninite stage
Modelling of conditions (pH, Eh and concentration)
during the formation and remobilization of uraninite
The dissolution or precipitation of solid uranium phases
can be modelled using a calculation based on known thermochemical and electrochemical parameters. Unfortunately, natural processes are extremely complicated in the Jáchymov ore district, influenced mainly by the following
quantities: U activity, pH, Eh, p, T, activities of O, S, CO2,HS, H2S, and AsH3, the ionic strength of solution, typeof crystal structure, the size of crystallites, a.o. In order toestimate the values of important variables (pH, Eh, T and169

U activity), it is necessary to introduce dramatic simplifications and assume limited effects of individual factors.
Chemical analyses of the Jáchymov uraninite confirm
a composition close to UO2, with a low component of UVI.
General assumptions for a proposed model: reaction inhydrothermal environment in the temperature range of150200 °C, moderately oxidizing, neutral or weakly basic conditions; solution contains CO2, HCO3, Cl, SO42,uranium as dominantly UVI carbonate complexes.
On the basis of these assumptions, Eh, pH diagram(Fig. 33) was designed using tabular data [457]. The diagram shows conditions for uraninite precipitation, basedon the calculation with amorphous UO2. The graph showsthat UO2 can coexist at a range of pHEh conditions withthe following ions: U(OH)22+, U(OH)5, UO2+. Activitiesof these ions are also influenced (in equlibrium) underconditions of the above defined environment, with a variable of pHEh conditions, by ions: U4+, U(OH)3+, UO22+,[UO2(CO3)3]4, [UO2(CO3)2]2, UO2CO3.

Fig. 33. pH-Eh diagram of equilibria of products of a reaction of uraninite with hydrothermal environment in the system:
amorphous uraninite  O2  CO2  H2O at 100 °C.

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Journal of the Czech Geological Society 48/34(2003)

T a b l e 1 . Equilibria concentrations in hydrothermal environment in the system: amorphous UO 2O2CO2H2O at 100 °C.
UO2 + 2H+ ↔ U(OH)22+
ÄG° (UO2) = –1031.8 KJ mol–1

pH = (–log [U(OH)22+] – 1.32)/2

ÄG° (U(OH)22+) = –992.4 KJ mol–1
UO2 + 3H2O ↔ U(OH)5– + H+
ÄG° (H2O)liq = –237.1 KJ mol–1

pH = log [U(OH)5– ] + 12.2

ÄG° (U(OH)5–) = –1641.8 KJ mol–1
UO2 ↔ UO2+ + e–
ÄG° (UO2+) = –968.6 KJ mol–1

Eº = 0.325 V
Eh = E° + 0.059 log [UO2+]
UO2+ ↔ UO22+ + e–

ÄG° (UO22+) = –952.7 KJ mol–1

E° = 0.165 V
Eh = E° + 0.059 log ([UO22+]/[UO2+])
UO22+ + H2O + CO2 ↔ UO2CO3 + 2H+
ÄG° = –46.2 KJ mol–1

ÄG° (CO2) = –394.4 KJ mol–1

pH = (log ([UO2CO3]/[UO22+]) – log pCO2 + 8.095)/2

ÄG° (UO2CO3) = –1538.0 KJ mol–1

UO2CO3 + H2O + CO2 ↔ UO2(CO3)22– + 2H+
ÄG° (UO2(CO3)22–) = –2106.8 KJ mol–1

ÄG° = –62.7 KJ mol–1
2–

pH = (log ([UO2(CO3)2 ]/[UO2CO3]) – log pCO2 + 11.25)/2
UO2(CO3)22– + H2O + CO2 ↔ UO2(CO3)34– + 2H+–
ÄG° (UO2(CO3)34–) = –2658.5 KJ mol–1

ÄG° = –79.8 KJ mol–1
pH = (log ([UO2(CO3)3

4–]/[UO2(CO3)22–]) – log pCO2 + 13.74)/2

ÄG° – standard free Gibss energy at 25 °C [457],
pCO2 – partial pressure of CO2 (atm)

The overlap area of horizontal and vertical belts in thepHEh diagram indicates the most probable range of conditions for the formation of the carbonateuraninite stageof mineralization. A detailed analysis of the overlap area
yields the set of equations shown in Table 1.
Based on thermodynamic relations obtained, concentrations of the individual above mentioned ions were calculated under equilibrium conditions such as to correspond to conditions at corners and the centre of thequadrilateral defined in Fig. 33. Inspection of data in Table 2 shows that the role of some ions in the interactionof amorphous UO2 in hydrothermal environment containing O2CO2H2O under variable redox and pH conditions
is not significant. This group of ions includes U(OH)5,
U(OH)22+, UO2+. Some of the reactions are independentof redox conditions, some of variation in pH (see Table 1
with equilibrium reactions).
The review also shows that a combination of a slightly
acid environment (pH = 5) with reducing conditions (Eh= 0.2 V) is infavourable for the transport of uranium.
Neutral conditions (Eh = 0.0 V, pH = 7) do not represent
optimum conditions for U transport either. In acid andoxidizing environment, uranium is mobile in the form of
UO22+ ions, or as undissociated dissolved UO2CO3.
Analyses of pH of hydrothermal quartz powdered insuspension [351] yielded values in the range of 7.08.4.
This indicates that hydrothermal environment during themain mineralization stages was probably moderately basic, with pH ~ 8. Based on these assumptions, the combination of Eh = 0.2 V, pH = 5 can be eliminated. A fun-T a b l e 2 . Equilibria reactions, thermochemical and electrochemical interpretation of products in the system: amorphous UO2O 2CO2H2 O
at 100 °C.
Eh = –0.2 V, pH = 5

Eh = 0.2 V, pH = 5

Eh = –0.2 V, pH = 9

Eh = 0.2 V, pH = 9

Eh = 0.0 V, pH = 7

ion

log (ai)ion

log (ai)ion

log (ai)ion

log (ai)ion

log (ai)

U(OH)5–
U(OH)22+UO2+UO22+
UO2CO3
UO2(CO3)22–
UO2(CO3)34–

–7.20
–8.68
–8.90
–15.08–15.18–18.43–24.17UO22+
UO2CO3
UO2+
UO2(CO3)22–
U(OH)5–
U(OH)22+
UO2(CO3)34–

–1.53
–1.62
–2.12
–4.87
–7.20
–8.68
–10.61

UO2(CO3)34–
UO2(CO3)22–
U(OH)5–
UO2CO3UO2+
UO22+
U(OH)22+

–1.00
–2.43
–3.20
–7.18
–8.90
–15.08–16.68UO2CO3
UO2(CO3)22–
UO2(CO3)34–
UO22+
UO2+
U(OH)5–
U(OH)22+

–1.00
–1.00
–1.00
–1.53
–2.12
–3.20
–16.68UO2(CO3)22–
UO2CO3
U(OH)5–
UO2(CO3)34–
UO2+
UO22+
U(OH)22+

–3.65
–4.40
–5.20
–5.39
–5.51
–8.31
–12.68ai – calculated equilibria activity of the i-th ion for given pH-Eh conditions and temperature of 100 ºC and pressure of 0.1 MPa.

Journal of the Czech Geological Society 48/34(2003)

damental condition for hydrothermal uranium formationis the deposition of U in the form of precipitated andeventually recrystallized gels to phases of a general composition (U 4+1-x-y-zU 6+ xREE3+yM 2+z v)O2+x-0.5y-z-2v) [459]through the reduction of mobile ions UVI [458]. Although
the existence of UVI is possible in slightly basic reducing environment (on the basis of the above calculations),
we suppose, due to the redox potential exchange, that theenvironment was slightly oxidizing.
Precipitation of UO2 and conditions of uraninite
formation
The above discussed calculations, analyses and assumptions indicate that uraninite in the Jáchymov ore district
formed from gels, later partly recrystallized, precipitated from slightly basic solutions (pH = 8) carrying ionsof UO2(CO3)34 and UO2(CO3)22. These ions play themost important role during the transport in hydrothermal
environment and in the precipitation of UO2 gels. The
trigger in UO2 precipitation was a change in pH and Ehcaused by the reaction of hydrothermal solutions withmaterials in the vein or the neighbouring wall-rock,which possibly created sufficient reaction potential andprovided for a change in pH; the concentration of
UO2(CO3)34 and UO2(CO3)22 ions is not directly dependent on Eh. The equations for precipitation of UO2 canbe generally formulated as follows:
UO2(CO3)22 + 2H+ + 2e ↔ UO2↓ + 2HCO3
E°(U VI/UIV) ≈ 0.2 V, estimation from [460]4
UO2(CO3)3 + 3H+ + 2e ↔ UO2↓ + 3HCO3
A quantitative course of these reactions depends on thefulfilment of the following conditions: reducing condition of sufficient capacity
 change in pH value allowing decomposition of uranylcarbonate complexes
 environment providing for pressure decrease and release of CO2
 environment containing sufficient Ca and Mg for the
crystallization of carbonates
Reducing conditions in the Jáchymov veins resulted
from Fe2+, contained in country rocks, vein carbonates orsulphides; reduction of organic matter played a negligible role owing to its low concentration. The oxidation ofsulphides and Fe2+ phases allowed a sufficient increase inacidity:
2Fe(OH)2 ↔ Fe2O3↓ + H2O + 2H+ + 2e
E°(FeIII/FeII) = 0.86 V [457]+HS ↔ S°(aq) + 2e + H
E°(S°/SI) = 0.133 V [403]2
2S2 ↔ 4S°(aq) + 4e
E°(S°/SI) = 0.30 V [461]2S ↔ S°(aq) + 2e
E°(S°/SII) = 0.48 V [403]171

In the second step, oxidation continued with the participation of H2O:
S°(aq) + 3H2O ↔ SO2(OH) + 5H+ + 4e
E°(SIV/S°) = 0.66 V [403]IV
2
S O2(OH) + H2O ↔ SO4 + 3H+ + 2e
E°(SVI/SIV) = 0.94 V [403]And finally, a shift in the redox equation UVI ⇒ UIV,
caused by the consumption of HCO3 ions via precipitation of Ca2+ and Mg2+, in favour of the final products calcite and dolomite:
2HCO3 + 2Ca2+ ↔ 2CaCO3 + H2O
4HCO3 + 2Mg2+ + 2Ca2+ ↔ 2CaMg(CO3)2 + H2O.
It is probable that the redox mechanisms discussed willvary and oscillate, which may result in local reversal of reactions toward the initial components. Thus with the onsetof the arsenicsulphide stage, with an increase in redox
potential and acidity, mechanisms started to operate, facilitating transition of U in solution via oxidation UIV ⇒ UVI:UO2↓ + 2H2O ↔ U3O8↓ + 4H+ + 4e
E°(U5.3/UIV) = 0.278 V (crystalline) [460]
E°(U5.3/UIV) = 0.031 V (amorphous) [460]
U3O8↓ + H2O + H+ ↔ 3 UO2(OH)+ + 2e
Eo(UVI/U5.3) = 0.707 V [460]
UO2↓ ↔ UO22+ + 2e
E°(UVI/UIV) = 0.221 V [460]
The ions UO22+ and UO2(OH)+ are very mobile, which
resulted in frequent rejuvenation (remobilization) of theoriginal uraninite.
The wall-rock alteration affected mainly Mg and Fe
minerals, Ca was probably transported by mineralizing
solutions. The oxidized forms of Fe were deposited mainly in the abundant hematite pigment in vein fillings aswell as in altered wall-rock.
Precipitation of Fe oxides
Carbonate crystals near uraninite contain thin layers richin Fe oxides, indicating growth zones. Formation of suchgrowth zones can be explained by the mechanism of redox reactions. Due to the oscillation in Ca2+/Fe2+ ratio andvarying Eh and pH, the successive growth zones containa variable FeII component. During the episode with lower
pH and increased oxidizing capacity of the solution, U inuraninite was dissolved at the formation of mobile UO22+:
UO2↓ ↔ UO22+ + 2e
Over a short distance, UO22+ ions were reduced backto UO2 through reaction with Fe2+, bound in a thin layerof ankeritic carbonate:
UO22+ + 2FeCO3 ↔ 2Fe3+ + 2CO32 + UO2↓

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Journal of the Czech Geological Society 48/34(2003)

The released Fe3+ ions were precipitated in a neutral
or slightly basic environment, buffered by carbonates,
following the reaction:

free energy and electromotoric strength (EMS) given by
the relation:∆GA° = n F E°,

2Fe3+ + 6(OH) ↔ 3H2O + Fe2O3↓

where F is Faraday constant and E° is standard reduction potential of the equations below. The following reactions hold for a neutral or slightly basic environment:

Fe3+ + 3(OH) ↔ H2O + FeO(OH)↓
In contrast to the precipitated and relatively stable
Fe2O3, the newly precipitated UO2 was easily soluble andre-mobilized over longer distances.
Redox potential of hydrothermal environment after
the carbonateuraninite stage

2S° + 2e ↔ (S2)2

E°(S°/SI) = 0.25 V [403]2As° + 2e ↔ (As2)2

E°(As°/AsI) ≈ 0.50 V(estimated value from [403])

3As + 2e ↔ (As3)2

E°(As°/As` / ) ≈ 0.33 V(estimated value from [403])!

The following model is proposed for the interpretationand prediction of the deposition of ore components in hydrothermal veins (arsenide and arsenicsulphide stages)
based on a change of redox conditions controlling mineral crystallization.
Estimates of Gibbs free energy were calculated for theoxidation of the anionic part of minerals as a basis for
estimates of reducing capacity or stability/instability under changing redox properties of the environment in dependence of Eh on hydrothermal solutions. The following equation was employed:
AN° + ne ↔ ANn,where AN represents anionic part of an arsenide, sulpharsenide or sulphide molecule.
In a formal way, Gibbs free energy is assigned to mean
the value of valence of atoms in anionic part of each molecule (n), based on the relation between the exchange of

Fig. 34. Sulphur vs. nickel content in selected arsenides of the arsenide
mineralization stage.

T a b l e 3 . Oxidation state and modified Gibbs free energies (ÄG°) for selected arsenides, sulpharsenides and sulphides of Ni, Co, Fe.
S

mineral

OS

n

E°

∆ G°

nickeline
löllingite
clinosafflorite
safflorite
rammelsbergite
skutterudite
nickel-skutterudite
arsenopyrite
alloklasgersdorffite
pyrrhotite
marcazite
pyritesternbergite
sphalerite
chalcopyrite
acanthiteproustitearsenic

3.0
1.0
1.0
1.0
1.0
0.7
0.7
1.0
1.0
1.0

1
2
2
2
2
3
3
1
1
1

1.37
0.50
0.50
0.50
0.50
0.33
0.33
0.50
0.50
0.50

132
97
97
97
97
96
96
48
48
48
0
0
0
0
0
0
0
66
0

3.0
0.0

1
1

0.68

OS

1.0
1.0
1.0
2.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0

n

1
1
1
8
2
2
3
1
2
1
3

Me
E°

0.25
0.25
0.25
0.48
0.25
0.25
0.48
0.48
0.48
0.48
0.25

∆ G°

Me nK

OS

0
0
0
0
0
0
0
24
24
24
371
48
48
139
46
93
46
72
0

Ni
Fe
Co
Co
Ni
Co
Ni
Fe
Co
Ni
Fe
Fe
Fe
Fe
Zn
Cu
Ag
Ag
As

3.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.3
2.0
2.0
2.5
2.0
2.0
1.0
1.0

1
1
1
1
1
1
1
1
1
1
7
1
1
3
1
2
2
3
1

OS  oxidation states ([592], [593], [594]),n, (nk)  anion (or cation) atoms per formula unit,
E° (V)  estimated standard reduction potential,ÄG° (kJ/mol)  free Gibbs energy (calculated from estimated standard reduction potentials).

results
Me OS

Ag

1.0

Fe

2.0

Σ∆
Σ∆G° AN Σ∆Σ∆G °AN /nK
132
97
97
97
97
96
96
72
72
72
371
48
48
139
46
93
46
7
0

132
97
97
97
97
96
96
72
72
72
53
48
48
46
46
46
23
2
0

grade of oxidation

AsJournal of the Czech Geological Society 48/34(2003)

173

The ∆GA° values express the ability of atoms to accept
electrons and increase their oxidation state. Minerals witha higher ∆GA° value would precipitate more readily in reducing conditions, and minerals with a lower value or corresponding to zero would preferentially crystallize in environment with a higher oxidation potential.
Mean values of valence (oxidation state) of all atoms
in anionic part of the arsenides molecule are generally
lower than in sulpharsenides and sulphides (see Table 3).
Application of the above calculations to the interpretation of crystallization succession of the five-element
assemblage in the Jáchymov ore district shows (Table 3)that the oxidation potential of the arsenide stage was very
low, probably the lowest among all the mineralization
stages. An important role during the arsenide stage wasplayed by the pH of the environment.
Also important is the role of sulphur [447], which did
not take part in mineral crystallization due to its low
fugacity (or high fugacity of As) at the beginning of thearsenide stage. Later, when S fugacity increased, sulphur
partially participated in the mineralization process. Interesting are significant sulphur contents (Fig. 34) in the arsenides (rammelsbergite, skutterudite) of the arsenide
stage. On the other hand, some sulpharsenides are deficient in S, due to the high As fugacity. This is seen in
gersdorffite with As/S ratio varying from the normal value 1 : 1 to extreme values of 1.85 : 0.15. Minerals of suchcomposition still retain gersdorffite structure and isotropic optical behaviour (see figures for gersdorffite).
Arsenide stage
Crystallization of silver and bismuth dendrites
Ag and Bi are delivered in the form of Cl complexes, mainly AgCl2 and BiCl4. The absence of Ag2S in the primarysilver accumulations indicates a deficiency in HS ions,which would preferentially precipitate sulphides [see below β(AgHS) » β(AgCl2)], in comparison with the chlorides. Besides this, free HS  ions in coexistence with
AgCl2 complexes spontaneously oxidize to SO42 ions:
HS + 4H2O + 8AgCl ↔ SO4 + 8Ag + 16Cl + 9H



2

2



+

Hydrothermal solutions were strongly saline (ca. 20 %CaCl2, NaCl), weakly acid (pH ~ 67), and S fugacity
was probably low.
Mercury shows similar chemical properties of chloride
complexes as those characteristic of Ag, resulting in joint
precipitation of the two metals. This is indicated by theoccurrence of mercurian silver with up to 2 wt. % Hg inrims of the old silver dendrites.

it is at 20 °C log(β2) = 5.14 [463], log (Ks(AgCl)) =9.75 [463]This gives equilibrium concentration [AgCl 2 ] =β2Ks[Cl] = 2.45 · 105 [Cl] mol/l. The concentration ofdissolved AgCl2 ions in the presence of 20 % and 0.1 %NaCl at 20 °C is 8.6 · 105 mol/l and 4.9 · 107 mol/l, respectively, of free Ag.
In case of a parallel existence of two hydrothermal
solutions with comparable contents of HS a Cl ions at
a given time, the HS ion will be preferentially instrumental in Ag transport. For the reaction
Ag+ + 2HS ↔ Ag(HS)2
is log(β2) = 17.43 [463], and for argentite (acanthite) dissociation:
Ag2S ↔ 2Ag+ + S2
is KS = 48.8 [463]. From this follows that [Ag(HS)2] =
β2[Ag+][HS], where [Ag+] = 3√2 Ks. For [HS] concentration of, e.g., 0.001 mol/l, [Ag(HS)2] = 0.014 mol/l.
Bismuth: For the reaction
Bi3+ + 4Cl ↔ BiCl4
at 20 °C, log(β4) = 5.52 is obtained [463]. This shows that
Bi in chloride solution will behave in a similar way as Ag.
Nevertheless, stability of the complex BiCl4 is strongly
influenced by Cl concentration and the value of pH:
BiCl4 + H2O ↔ BiOCl(s) + 2H+ + 3Cl, where log K =1.20 [463].
Growth of native Ag and Bi dendrites
As noted above, spontaneous precipitation of native Ag
or Bi in the form of dendrites proceeded by the reduction of Ag or Bi chloro-complexes:
AgCl2 + e ↔ Ag + 2ClBiCl4 + 3e ↔ Bi + 4ClSilver: For the reaction

The rapid growth of skeletal crystals was caused byincreased concentration of [Ag] ~10-5 mol/l (a similar
situation applies to Bi), and the existence of reducing
environment. The latter was represented in this mineralization stage mainly by biotite-bearing rocks withincreased FeII and by rocks with finely dispersed pyrite.
The released Fe2+ in neutral or slightly basic environment is readily oxidized to Fe2O3. In an oxidation environment, the oxidation of pyrite readily takes place following the equation:

Ag+ + 2 Cl ↔ AgCl2

FeS2 + 8H2O ↔ Fe2+ + 2SO42 + 16H+ + 14e.

Estimate of concentration of mobile Ag or Bi ions:174

Dendritic silver and bismuth crystals precipitated rapidly at separated sites. The following factors are considered important for their separated precipitation:
 difference in standard electrochemical potential
E°(Bi3+/Bi) = 0.320 V, E°(Ag+/Ag) = 0.799 V.
 differences in the stability of AgCl2 and BiCl4 complexes in hydrothermal environment. The anion BiCl4
readily changes to BiOCl with a low solubility.
Precipitation of NiCoFe polyarsenides
Precipitation of polyarsenides took place under increasingly reducing conditions imposed by high arsine (AsH3)
contents. Arsine is dissolved in brine fluids; dissociation
to ions AsH2 (log(K) ≈ 155 [425]) is negligible.
Batches of fluids containing Ni, Co, Fe were closely
tied to AsH3, at fairly low S fugacity, in a neutral or
weakly acid environment. Under these conditions Ni, Co,
Fe aqua chloro-complexes formed, facilitating the transport of these elements. Complexes with ligand AsH3 were
probably also formed: [Ni(H2O)u(AsH 3)vXw]2w, where
u+v+w = 4 or 6, X = OH, Cl; co-ordination of the central atoms (Ni,Co,Fe) of the above group of complexes
is 4 or 6, with co-ordination 6 being the most common.
The disintegration of these complexes, caused mainly
by a change in pressure and temperature, resulted in the
precipitation of arsenides (polyarsenides in general). The
appearance, chemical composition and physical properties
of some arsenide aggregates suggest a rapid precipitation,
which indicates high concentrations of the complexes.
The formation of arsenidesdiarsenidestriarsenides isclosely related to the redox potential of the environment.
Based on the redox model (see Redox potential of hydrothermal environment after carbonate-uraninite stage
above), it is possible to say that the redox potential values play a leading role in the precipitation, and control
the type of arsenide to be precipitated. Our observationsof several generations of individual arsenides indicate repeated changes in redox environment. Local variation inavailable oxidants, such as UO22+, Fe3+, O a.o., may havecaused these changes.
Arsine oxidation in acid environment can proceed easily through the reaction with wall-rocks (Fe3+, Mn3+) andwith components brought by the mineralizing solutions(UO22+, O a.o.). Small interposed layers of uraninite separating some arsenide layers serve as evidence of the participation of UO22+ ions in the deposition of polyarsenides.
The chemical analyses of arsenides (in particular triarsenides) reveal a variable composition (Ni, Co, Fe) of
successive growth zones. Hydrothermal solutions of thefirst stage show the highest contents of metal in the order of Ni > Co ≥ Fe, and, at the same time, the lowest
values of Eh potential, thus providing for the crystallization of nickeline, with the highest content of metals andwith the lowest valency state of As:

Journal of the Czech Geological Society 48/34(2003)[Ni(H2O)u(AsH3)vXw]2w ↔ NiAs + 3H+ + wCl + uH2O
+ (w1)AsH3 + eor in a simplified form (mild oxidation):
Ni2+ + AsH3 ↔ NiIIIAsIII + 3H+ + eThe deposition of diarsenides and triarsenides proceeded in a similar manner only in the environment with anincreased Eh potential. The following equations express
multistage redox processes:
Ni2+ + AsH3 + H2S ↔ NiIIAsISI + 5H+ + 3e
Ni2+ + 2AsH3 ↔ NiIIAsI2 + 6H+ + 4e
Ni2+ + 3AsH3 ↔ NiIIAsII/III+ 9H+ + 7e
3Oxidation states of As and Ni are adopted from [592],[593], and [594].
Variation in As fugacity and other parameters in thecourse of the arsenide stage is shown in Fig. 1. Increasing acidity, not significantly buffered by carbonates, ischaracteristic of these processes. The increased acidity ofsolutions resulted in the alteration of wall-rocks and breccias, characterized by the formation of clay minerals (seefigure for kaolinite in the encyclopaedic section).
In a general way, the experimentally confirmed trend
in the precipitation of arsenides corresponds to a gradually increasing oxidation potential of the whole fracture
system, following the succession:
arsenides ⇒ diarsenides ⇒ triarsenides,
Oscillations in the redox potential resulted in a partial repetition of this succession. The above equations ofarsenide precipitation indicate increasing acidity in theprocess.
Antimony was probably transported in the form of stibine (SbH3), in a similar way as arsenic in the form of AsH3.
In the final stage of polyarsenide precipitation, the fugacity of Sb increased due to the supply of additional Sb.
Dissolution of Ag and Bi dendrites
Dendritic crystals of Ag and Bi were quantitatively
overgrown by Ni, Co, Fe polyarsenides and some otherminerals. After the deposition of polyarsenides, the dendritic aggregates of native Ag and Bi were nearly quantitatively dissolved.
It appears that the dissolution of Ag and Bi was notaccomplished purely by chemical dissolution (metals
were surrounded by polyarsenides), but rather throughelectrochemical dissolution. Metallic Ag or Bi enclosed
in non-conducting polyarsenides behaved as an electro-

Journal of the Czech Geological Society 48/34(2003)

chemical half-cell in solutions with a high content of Cl
ions. The potential of such half-cell is:Ag(s) + Cl ↔ AgCl(s) + eE°1(Ag /Ag) = 0.222 V.
+

175

Ag + HS ↔ AgHS
Ag + 2HS ↔ Ag(HS)2

Released AgCl in such environment is dissolved:

Increased acidity results in the precipitation of Ag2S
(argentite):

AgCl(s) + Cl ↔ AgCl2(aq)

2Ag(HS)2 + 2H+ ↔ Ag2S + 3H2S.

To achieve electrolysis resulting in the dissolution of Ag,
the second part of the electrochemical cell must represent
a redox half-cell with a potential higher than 0.222 V. With
regard to ions available in the hydrothermal environmentat Jáchymov, the following reaction is considered:

Precipitation of Ag2S is also triggered by an increased
redox potential:

Fe3+ + e ↔ Fe2+

E°2(Fe3+/ Fe2+) = 0.771 V

O2 + 4H+ + 4e ↔ 2H2O

E°2(O/ O2) = 1.229 VThis redox reaction applies only to a slightly acid environment. The oxidation ability of UO22+ is insufficientfor the dissolution of Ag: E°(UVI/UIV) = 0.221 V.
The resulting electrochemical cell:Ag(s) + Cl + Fe3+ ↔ AgCl(s) + Fe2+

4H2O + Ag(HS)2 ↔ Ag2S + SO42 + 10H+ + 8e.
Precipitation of silver sulphosalts (proustite, xanthoconite) proceeds through the reaction of Ag with complex ions H2As3S6 or H2AsS3 [590]:
9AgHS + H+ + H2As3S6 ↔ 3Ag3AsS3 + 6H2S
These reactions take place owing to increasing
S fugacity, redox potential and acidity of the hydrothermal environment.
Dissolution of Bi was accomplished by similar electrochemical processes as in the case of Ag.

will generate under standard conditions potential of∆E° = E°2  E°1 = 0.549 V (see Fig. 35). Assuming thatthe cell will operate under connection of the electrodes by salt solution, Ag will be dissolved and ∆Govalue will be negative: ∆G° = nF∆E° = 52.9 kJ/mol.
To obtain activity relation [Fe3+]/[Fe2+], lets assume that
∆G = 0 = ∆G° + RT ln(K), resulting in ∆G° = RT ln(K)= RT ln([Fe2+]/[Fe3+]). For the reaction to proceed (∆G°< 0), it is necessary to have [Fe2+]/[Fe3+] = exp(∆G°/RT) = 3.5 · 10 5. This condition will be satisfied underhydrothermal conditions in a certain range of pH values.
Hydrolysis of Fe3+ ions is proceeding following the reaction:
Fe3+ + H2O ↔ Fe(OH)2+ + H+
log(K1) = 2.20, log(Ks) = 36.5 [463];
this implies that approximately [Fe3+] = ([H+ ]cFe)/K,
where cFe is the total content of dissolved Fe. For example, for pH = 6, cFe = 102 mol/l will be [Fe 3+] =1.6 · 105 mol/l.
In the course of Ag dendrite dissolution, Ag is mobilized and deposited in other parts of the ore veins. It becomes bound mainly in sulphides and sulpharsenides,
only to a limited degree it is precipitated as native Ag.
In the transport of Ag from dissolved dendrites, HS ionsplayed probably a much more important role in the formation of stable complexes AgHS and Ag(HS)2, wherelog(β1) = 13.48 and log(β2) = 17.43 [589]Fig. 35. Schematic image of electrodissolution of native silver.

Arsenicsulphide stage
Precipitation of arsenic
Arsine-rich solutions, after removal of metallic ions (Ni,Co, Fe), quantitatively precipitated native As under moderately acid conditions (pH ~ 6) in an oxidizing environment. Sulphur fugacity was insufficient for the productionof AsS phases. Realgar was observed only as insignificant accumulations in the association with arsenic.

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Journal of the Czech Geological Society 48/34(2003)

The following relation applies in an acid environment:
AsH3 ↔ As° + 3H+ + 3e
E°1(As°/AsIII) = 0.225 V [403].
The reaction can take place spontaneously, if under standard conditions ∆G° of the whole reaction is negative, i.e.
∆E° > 0. If ∆E° = E2°  E1°, where E2° represents redox
reaction, where E°2 > E°1, i.e., for E°2 > 0.225 V, oxidation of arsine to As will take place. In other words, arsineoxidation in acid environment will proceed easily [591],
in particular with Fe3+, UO22+, O2 and other oxidants characterized by a positive standard redox potential.
Precipitation of base metal sulphides andsulpharsenides

Fig. 38. J167P/1. 1  bismuth, 2  BiSMo, 3  bismutite, 4  quartz.
Eliá mine, 180 m level, 2A vein. BSE image. Magnification 540x.

Arsenic fugacity abruptly decreased during this period
but S fugacity was gradually increasing. Redox poten-

Fig. 36. J116P/D-1. 1  AgFe-sulphides containing (Ni, As),2  pyrite, 3  nickelskutterudite. Svornost shaft, 8th level, Geschieber
vein. BSE image. Magnification 540× .

Fig. 39. J167P/4. 1  bismuth, 2  BiSMo, 3  bismutite, 4  quartz.
Eliá mine, 180 m level, 2A vein. BSE image. Magnification 270× .

Fig. 37. J141P/D. 1  bismuth, 2  Ag-millerite, 3  argentite,
4  proustite, 5  pyrite, 6  annabergite. Eliá mine. BSE image.
Magnification 130×.

Fig. 40. J171P/D. 1  silver, 2  argentite, 3  argentopyrite, 4  löllingite, 5  dolomite. Zimní Eliá mine. BSE image. Magnification 160×.

Journal of the Czech Geological Society 48/34(2003)

177

Fig. 41. J171P/F. 1  silver, 2  silver, argentopyrite and pyrargyrite,
3  pyrargyrite, 4  löllingite, 5  dolomite. Zimní Eliá mine. BSE
image. Magnification 27×.

Fig. 42. J171P/F-1. 1  silver, 2  pyrargyrite, 3  argentopyrite,
4  chalcopyrite, 5  argentopyrite decomposed into acanthite andpyrite. Zimní Eliá mine, dump. BSE image. Magnification 320× .

tial and acidity were increased in fractures of the vein
system.
Minerals containing metallic elements (Cu, Ag, Pb,
Zn, Fe) crystallized. The first generation precipitatedfrom chloride or hydrosulphide complexes of hydrothermal solutions. Crystallization was triggered by temperature decrease, for example:
PbCl3 + H2S ↔ PbS + 3Cl + 2H+
ZnCl42 + H2S ↔ ZnS + 4Cl + 2H+
2AgHS ↔ Ag2S + H2S
precipitation was positively influenced by increasing acidity, for example:
2Ag(HS)2 + 2H+ ↔ Ag2S + 3H2S
9MeIHS + H+ + H2As3S6 ↔
3MeI3AsS3 + 6H2S
Redox potential of the arsenic
sulphide mineralization stage:oxidation states of arsenic,
sulpharsenides and Ag, Cu
sulphides
The majority of elements in mineralsof this paragenesis have a higher oxidation state compared to the arsenide stage. A significant participation of sulphur, and to a lesser extent
antimony, is observed in this mineralization stage.
The oxidation state of As changed
from III or I to 0 and in part to +III,

Fig. 43. J171P/F-2. 1  silver, 2  pyragyrite, 3  argentopyrite,
4  chalcopyrite, 5  argentopyrite decomposed into acanthite andpyrite. Zimní Eliá mine. BSE image. Magnification 720×.

T a b l e 4 . Values of sulphur fugacity [log fS2 (atm.)] of selected minerals of Jáchymov
district, calculated for temperatures of 60200 °C.
sample

60

80

100

120

140

160

180

200

vaesite–22.5 –20.7 –19.0 –17.5
realgar–22.7 –20.9 –19.3 –17.8
xanthoconite/argentite –22.9 –21.2 –19.7 –18.3
proustite/argentite–23.6 –21.8 –20.1 –18.7
argentite–24.0 –22.4 –21.0 –19.8
acanthit–24.6 –22.9 –21.4 –20.1
bismuthinite–26.3 –24.3 –22.6 –21.1
stibnite–27.4 –25.2 –23.3 –21.6
matildite–27.2 –25.2 –23.5 –21.9
covellite–28.1 –26.1 –24.3 –22.7
argentopyrite–26.5 –24.9 –23.6 –22.4
pyrite/pyrrhotite–31.3 –28.7 –26.3 –24.2
pyrite–36.5 –33.8 –31.5 –29.3
chalcocite–38.5 –36.0 –33.8 –31.9
chalcopyrite–41.5 –38.7 –36.2 –34.0
galena–41.5 –38.8 –36.3 –34.0
pyrrhotite–41.9 –39.2 –36.7 –34.5
molybdenite–43.9 –40.8 –38.0 –35.4
sphalerite–70.3 –65.7 –61.6 –58.0
Data were calculated from ÄGº values published in [449].

–16.2
–16.5
–17.1
–17.4
–18.7
–18.9
–19.7
–20.0
–20.5
–21.3
–21.2
–22.3
–27.4
–30.1
–31.9
–32.0
–32.5
–33.2
–54.7

–15.0
–15.3
–16.0
–16.2
–17.6
–17.8
–18.4
–18.6
–19.2
–20.0
–20.2
–20.6
–25.7
–28.5
–30.1
–30.2
–30.7
–31.1
–51.7

–13.9
–14.2
–14.9
–15.1
–16.7
–16.8
–17.2
–17.3
–18.0
–18.8
–19.3
–19.0
–24.1
–27.1
–28.4
–28.5
–29.0
–29.2
–49.0

–12.8
–13.2
–14.0
–14.1
–15.8
–15.9
–16.2
–16.1
–16.9
–17.7
–18.5
–17.6
–22.6
–25.7
–26.9
–27.0
–27.5
–27.5
–46.5

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Oxidizing environment is also indicated byhematite pigmentation in dolomitic carbonate, which is the main gangue mineral of thearsenicsulphide stage.
Role of sulphur in the hydrothermal
process
Nearly all stages of mineralization include
some sulphides, mainly pyrite and sphalerite. This shows that sulphur fugacities in thehydrothermal solutions were slightly abovezero.
It is assumed that sulphur was transported in solutions as HS, H2S or in anioniccomplexes with metallic elements[MeI(HS)n]n1 or [MeII(HS)n]n2. Relations between these components were influenced bymany factors, including pT, pH, Eh, contents of metallic elements, oxygen, etc., andregulated the dissolution or precipitation ofsulphides, sulpharsenides and sulphosalts. A
major factor triggering crystallization ofthese species was sulphur fugacity (fS2). Table 4 presents S fugacities for common sulphur-bearing minerals of all mineralization
stages. The relations are documented inFig. 44. The reference temperature of 160 °C
represents average temperature of all mineralization stages. The succession of minerals
from bottom to top (Table 4) generally corresponds to the formation of S-minerals in
the Jáchymov ore district. The first minerals to crystallize were sphalerite, pyrrhotite
and galena, the late minerals include sulphosalts and Bi-, Ag-sulphides.

Fig. 44. Sulphur fugacity (log fS2) vs. temperature for selected minerals of the
Jáchymov district.

resulting in accumulations of native arsenic, Ag, Cu sulphosalts with the anion [AsIIIS3]3 or of realgar. At the endof the arsenicsulphide stage, the activity of AsH3 significantly decreased (see Fig. 1, 3643).
Hydrothermal solution supplied some Sb jointly withdominant As, resulting in the crystallization of Ag, Pb,
and Cu sulphosalts with the anions [SbIIIS3]3, [SbIIIS2],[Sb4S13]14, [SbS45], a.o. The fairly increased Sb content
in the Hildebrand vein is responsible for the formationof Sb paragenesis: antimony, stibnite, arsenic, dyscrasite,
miargyrite, pyrargyrite, robinsonite, stibarsen, and intermetallic phases AsSb and BiSb.
Manifestations of increased oxidation potential areseen also in the deposition of silver. The main part of silver is bound as Ag+ cation in the above named sulphosalts, or as acanthite, argentopyrite or sternbergite.

GRAPHIC INTERPRETATION OF
CHEMICAL ANALYSES 
SUBSTITUTION TRENDS

This chapter gives interpretation of chemical analyses (forsource data see Primary minerals  encyclopaedic part) of
the most common ore minerals of the Jáchymov district.
Attention is focused mainly on minerals of the arsenide mineralization stage. Repeated variations in thecomposition and Eh potential of the mineralizing solutions during the deposition of minerals of this stage resulted in the crystallization of minerals with highly variable composition. Ni/Co/Fe and S/As ratios show
isomorphous substitutions in mineral structures [490].
Owing to the high fugacities of As, minerals with extremely high contents of As crystallized. This concerns
mainly pyrite containing nearly 11 wt. % As and gersdorffite containing over 62 mol. % As. In spite of thesechemical differences, the minerals retain their crystal
structure and optical properties (isotropic behaviour).

Journal of the Czech Geological Society 48/34(2003)

179

Group of diarsenides and triarsenides of NiCoFe

Fig. 45. NiCoFe ratio (atomic values) in selected diarsenides of thearsenide mineralization stage.

Fig. 48. Me (Me=Ni,Co,Fe) (apfu) vs. As (apfu) for nickel-skutterudite.

Fig. 46. NiCoFe ratio (atomic values) in selected triarsenides of thearsenide mineralization stage.

Fig. 49. Me (Me=Ni,Co,Fe) (apfu) vs. S (apfu) for skutterudite.

Fig. 47. Me (Me=Ni,Co,Fe) (apfu) vs. S (apfu) for nickel-skutterudite.
Regression lines express correlation trends of analytical data.

Fig. 50. Me (Me=Ni,Co,Fe) (apfu) vs. As (apfu) for skutterudite.

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Journal of the Czech Geological Society 48/34(2003)

Fig. 51. Me (Me=Ni,Co,Fe) (apfu) vs. S (apfu) for rammelsbergite.
Regression lines express correlation trends of analytical data.

Fig. 54. Me (Me=Ni,Co,Fe) (at.%) vs. As (at.%) for safflorite.

Fig. 52. Me (Me=Ni,Co,Fe) (at.%) vs. As (at.%) for rammelsbergite.

Fig. 55. Me (Me=Ni,Co,Fe) (apfu) vs. S (apfu) for löllingite. Regression lines express correlation trends of analytical data.

Correlation of chemical composition and structuralparameters of diarsenides

Fig. 53. Me (Me=Ni,Co,Fe) (apfu) vs. S (apfu) in safflorite. Regression lines express correlation trends of analytical data.

From the mineralogical point of view, crystal shapes of Nilöllingite (sample J171P) and Fe-safflorite (Fig. 57) similar to six-pointed stars are very interesting. The crystals are
triplets with twinning on plane (101) [348]. This type oftwinning is limited by the condition a/c = tan(60), wherea, c are unit-cell parameters. This condition is shown in
Fig. 56 by a grey band. The a/c ratio is a function of chemical composition  substitution of Ni, Co, Fe, so that thecondition for the formation of a hexagram (Fig. 58) is met
only for particular compositions, with löllingite containing
substantial Ni or safflorite containing Fe. These conditions
are often met in Jáchymov (see NiCoFe ternary diagramsfor safflorite and löllingite  Fig. 45). Triplets do not occur with rammelsbergite, since its a/c ratio (near 1.33) isJournal of the Czech Geological Society 48/34(2003)

far from the value of tan(60). Triangular symbols in the diagram below, plotting outside the grey belt, correspond to
relatively pure löllingite without triplets.

Fig. 56. a/c ratio of unit-cell parameters vs. Ni (in löllingite) and Fe(safflorite) (apfu). Grey belt shows range of a/c values compatible withformation of six-pointed star triplets.

181

Variation of Ni, Co, Fe against As in selectedpolyarsenides and sulpharsenides

Fig. 59. As (at%) vs. atomic ratio Ni/(Ni+Co+Fe) for selected minerals of the arsenide mineralization stage.

Fig. 57. J171P. Ni-löllingite triplets on (101) plane in the shape of sixpointed star. Zimní Eliá mine. SE image. Magnification 1450× .
Photo A. Gabaová.
Fig. 60. As (at%) vs. atomic ratio Co/(Ni+Co+Fe) for selected minerals of the arsenide mineralization stage.

Fig. 58. Computer model of löllingite triplet with twinning plane (101),in projection on ac plane. Unit-cell parameter ratio a/c = 1.732.

Fig. 61. As (at%) vs. atomic ratio Fe/(Ni+Co+Fe) for selected minerals of the arsenide mineralization stage.

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Group of sulpharsenides (gersdorffite, arsenopyrite,
alloclasite)

Fig. 65. Me (Me=Ni,Co,Fe) (at.%) vs. As (at.%) for gersdorffite.

Fig. 62. S vs. As (apfu) in selected sulpharsenides of the arsenide mineralization stage.

Fig. 66. Gersdorffite. Me (Me=Ni,Co,Fe) (apfu) is correlated withatomic ratio As/(As+S)

Fig. 63. NiCoFe ratio (atomic values) in selected sulpharsenides of
the arsenide mineralization stage.

Group of disulphides (pyrite, marcasite, vaesite)

Fig. 67. Me (Ni,Co,Fe) (at.%) vs. As (at.%) for pyrite.

Fig. 64. SAsNi ratio (atomic values) in gersdorffite.

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183

Group of tetrahedritetennantite (tennantite,tetrahedrite, argentotennantite, freibergite)

Fig. 68. As (apfu) vs. S (apfu) for pyrite.

Fig. 71. Sb (apfu) vs. As (apfu) diagram for tetrahedrite group minerals.

Fig. 69. NiCoFe ratio (atomic values) in pyrite of selected mineralization stages

Fig. 70. FeNiAs ratio (atomic values) in pyrite, marcasite, andvaesite.

Fig. 72. Zn (apfu) vs. Fe (apfu) diagram for tetrahedrite group minerals.
Argentotennantite and freibergite are Fe dominant members; two types(Fe and Zn dominant) of tetrahedrite are observed and FeZn solid
solution in tennantite was found.

Fig. 73. Ag (apfu) vs. As (apfu) diagram for tetrahedrite groupminerals. The minerals are separated into two group  Ag-free andAg-rich; no intermediate member was found.

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mawsonite demonstrate that these minerals form a stableassociation. This is in agreement with [514] but it is difficult to reconcile with the observation of [510] who described the association of stannoidite with stannite as a stable one. The occurrence of cassiterite in this mineral
assemblage indicates a higher activity of O2 during thisstage.
Arsenide stage

Fig. 74. Ag (apfu) vs. Cu (apfu) diagram for tetrahedrite group minerals.
There are two groups of analyses: Ag-free tennantites and tetrahedrites
and Ag-rich (in the range 35 Ag apfu) freibergites andargentotennantites.

DEDUCING THE CONDITIONS OF FORMATION
OF SELECTED SULPHIDESThe Jáchymov ore veins represent a multicomponent natural ore system. Individual stages of its development differ in their age, chemistry and pT conditions. Furthermore, the pT conditions varied within the individual
mineralization stages, and the mineralizations formed
during different stages overlap in space. Consequently,
it is often difficult to assign individual minerals to a specific mineralization stage. In the ore assemblages studied, disequilibria are very common, as is manifested by
the occurrence of zoned crystals, reaction rims, colloform
structures and the presence of various metastable assemblages. A low-temperature re-equilibration of higher-temperature ore assemblages is visible in exsolution intergrowths. Likewise, a change in the chemistry ofore-bearing fluids is preserved in the form of replacement
textures. All of these effects make the estimation of thepT conditions of formation a difficult task.
SnW sulpharsenide stage
This is a high-temperature stage related to the processesof greisenization of the granite that underlies the Jáchymov ore district. Chalcopyrite, stannite, sphalerite, kësterite, and roquesite formed broad solid solutions under the(high) temperatures that prevailed during their primary
crystallization that, in turn, gave rise to typical breakdown
textures formed during a subsequent period of cooling. Unfortunately, low-temperature solvi in the corresponding
synthetic systems have not been studied. Nevertheless, thepresence of mawsonite in this mineralization stage indicatesthat the temperature of formation could not have exceeded 390 °C. Considering also other evidence, the temperature of formation of minerals in this stage can be estimated at ~300 °C. Frequent intergrowths of stannite withThis stage is characterized by the presence of native Ag,
Bi (Hg) and of CoNi(Fe) arsenides and sulpharsenides.
According to the presence of either silver or bismuth,
this stage is further subdivided into silver and bismuth
parageneses. The presence of native metals proves avery low activity of sulphur, certainly below that of theunivariant sulphidation curves of Ag/Ag2S, Bi/Bi2S3 andHg/HgS. The observed succession of CoNi(Fe) arsenides ⇒ diarsenides ⇒ triarsenides corresponds to a
gradually increasing activity of arsenic during this stage.
The appearance of bismuthinite and bornite in the latephases of the arsenide stage documents an increasingsulphur activity, above that of the univariant curves bismuthinite/bismuth and bornite+pyrite/chalcopyrite.
Also, the presence of younger sulpharsenides with elevated contents of sulphur indicate a higher activity ofsulphur in the later phases of this stage. For example,
sulphur contents in the studied rammelsbergite are higher than the maximum content (1 wt. %) determined experimentally by [490], and vary within the range of
0.0219 wt. % whereas in pararammelsbergite, the sulphur contents reach only 0.082 wt. %. The analyses
also point out the unexpected existence of a rammelsbergitegersdorffite isomorphous series. The existenceof the same solid solution has recently been reported by
Fanlo et al. [587] from the San Juan de Plan deposits
in Spain. The variation in the As/S ratio in gersdorffite
and rammelsbergite, also echoed in arsenopyrite, reflects the variation in sulphur activity during their formation. Unfortunately, no experimental data are available and therefore it is not possible to use the As/S ratioof gersdorffite and rammelsbergite (and arsenopyrite,[503]) as a geothermometer. Fortunately, the occurrenceof skeletal aggregates of bismuth crystals can be used
as a fixed-point geothermometer; the crystals must havegrown below the melting point of Bi (271 °C). Likewise,
the occurrence of argentite indicates a temperature
above 179 °C. Rammelsbergite is an interesting mineral, because it is stable above 590 °C and inverts topararammelsbergite at lower temperatures. However,
any temperatures near 590 °C appear unacceptably high,but it should not be forgotten that the possible effect of
the activity of sulphur on the temperature of this phasetransition remains unknown. Still, this disqualifies rammelsbergite from the arsenic stage as a fixed-point
geothermometer. Karup-Møller and Makovicky [395]pointed out a topotactic influence of the matrix on theJournal of the Czech Geological Society 48/34(2003)

crystallization of rammelsbergite. Consequently, we canonly roughly estimate the temperature of formation as
being below 300 °C. Roseboom [512] studied thepseudoternary system of NiAs 2CoAs2FeAs2 and reported a wide solid solution with a miscibility gap between NiAs 2 and FeAs 2. This miscibility gap corresponds to the analyses of diarsenides from Jáchymov.
Again, no experimental data exist on the low-temperature solvi in this system that would allow us to narrow
down the possible range of temperature.
Arsenicsulphide stage
In this stage, the sulphur activity varied very widely.
Native metals are represented by arsenic, dendritic aggregates of silver, native antimony, and intermetallic alloys dyscrasite, AsSb and BiSb. Native silver and antimony indicate a sulphur fugacity below that of theargentite/silver and stibnite/antimony univariant curves(Fig. 44). On the contrary, minerals such as realgar (stable below 265 °C [507]), proustite and robinsonite together with a sulphur-rich rammelsbegite and safflorite
indicate that the sulphur activity must have been abovethat of the pyrite/pyrrhotite univariant curve (Fig. 44).
Also, several sulphides are present that have a limited
field of thermal stability and thus could be used as
fixed-point geothermometers. Xanthoconite is stable at
up to 191 °C, and transforms to the high-temperaturephase of proustite above this temperature. Stromeyerite
is stable at temperatures below 93 °C, whereas the presence of argentite and pyrargyrite indicates temperaturesof formation above 197 °C [506] and 192 °C, respectively [444]. In addition to that, stenbergite has a narrow field of thermal stability and breaks down at 152 °C
to argentite, pyrite and pyrrhotite [445], [508]. Proustite and pyrargyrite form a complete solid solution atabove 300 °C, but have a miscibility gap at lower temperatures [513], [347]. The lamellar intergrowths ofpyrargyrite and proustite thus indicate temperatures offormation below this temperature, even though the relevant solvi remain unknown. All these fixed-point geothermometers indicate low temperatures of formation,
probably in the range of 100200 °C.
Sulphide stage
In the sulphide stage, the main minerals are simple sulphides: chalcopyrite, sphalerite, galena, pyrite, accompanied by sulphosalt robinsonite. Löllingite, tennantite
and arsenian pyrite are the principal bearers of arsenicin the stage. The broad stability fields of these minerals do not permit to use these minerals as fixed-point
geothermometers. Only the presence of bornite withexsolution intergrowths of chalcopyrite indicates a higher temperature of formation which, unfortunately, cannot be quantified any closer. Low-temperature Cu (Fe)sulphides with narrow fields of thermal stability represent late phases of the sulphide stage. These minerals

185

indicate temperatures of formation below 100 °C.
Djurleite breaks down at 93 °C, anilite at 87 °C [507],
and smythite is reported to be stable below 75 °C [554].
The presence of sulphosalts is a consequence of a higher
sulphur activity in this stage (Fig. 44). A very low ironcontent of sphalerite furnishes evidence of the high activity of sulphur during the crystallization (assuming a
low pressure of formation and the presence of iron in
the system).
Conclusions
The hydrothermal vein systems at Jáchymov were formed
under a relatively low sulphur activity and at low temperatures  otherwise the mineral assemblages would contain no native metals and sulphides with a low thermal
stability. During the late phases of individual mineralization stages, the sulphur activity increased and the temperature decreased. Sulphosalts and sulpharsenides started
to crystallize in these late phases.
A feature common to all mineral assemblages is the
low activity of oxygen: with the exception of cassiterite(in the SnW sulpharsenide stage), no metal oxides arepresent in the studied ore assemblages.
PARAGENETIC RELATION OF PRIMARY AND
SECONDARY MINERALS
Compared to secondary minerals of the Jáchymov ore district, its primary mineralization is fairly simple in its ore
minerals as well as gangue minerals. Combinations of major primary minerals occur practically in the whole vertical range of the veins. However, this is not exactly valid
for all veins and in the whole district [475], [476], [477].
A typical scheme of a vein deposit with a leached oxidation zone, enriched oxidation zone, and cementation
zone above unchanged primary zone, is seen only on Geister and Geschieber veins and partly also on the Èervené
veins, which reach up to the surface. Majority of accessible surface outcrops of other veins show a relatively smallzone of nearly complete leaching of ore components, with
only sporadic occurrences of secondary uranium minerals(particularly torbernite, autunite and also uranophane andkasolite), bismuth minerals (bismutite, bismutoferrite),copper (malachite, mixite) and lead minerals (anglesite, pyromorphite). Even the relatively limited accumulations ofsecondary minerals may be leached, as indicated by limonite pseudomorphs after uranium micas. Majority of theveins do not crop out on the surface. Upper parts of veins
were removed by erosion and only middle and lower partsof veins remained.
The enriched cementation zone exploited in 16th century contained mainly silver, acanthite and chlorargyrite.
As noted by Mathesius [308], the richest accumulationsof ores were located at a depth of 6080 m. The fact thaturaninite reputedly occurred mainly at lower levels may
be due to its better solubility in the descending meteoritic water during supergene evolution.

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Fig. 75. Schematic vertical distribution of the five-element formation (AgBiCoNi±U) in a midnight vein.

A weak limonitization was limited to parts of veins
immediately reaching the land surface.
The richest secondary mineral assemblage formed under the recent or historically imposed conditions. The occurrence of the same or similar secondary minerals depends mainly on the content of the ore vein. This trend
is seen nearly in the whole vertical range of the veins.
The quantity of circulating fracture water and the modeof exposure of the ore vein by mining play the main role.
Subrecent minerals formed as a result of long-lasting
mining activity.

Alteration of bismuth minerals

Weathering processes induced by mining activity

Alteration of arsenic

Underground exposures of massive ores (including bismuth, nickel-skutterudite, pyrite, arsenic and uraninite)
carry minor coating of powdery secondary minerals. Richoccurrences of recently formed secondary minerals were
found in places where the ore was widely dispersed by
explosions. Rich accumulations of secondary minerals
were found in parts of ore-free adits, where fine-grained
part of ore from overturned tub was spilled on the floor(e.g., lindackerite, geminite at the 7th (Werner) level ofthe Rovnost I shaft). Rich accumulations of zýkaite were
found on disintegrating mica schist containing arsenopyrite. Generally, occurrences of secondary uranium minerals are common in the proximity of water drainage
ditches as coatings on barren rocks in adits, including
minerals like èejkaite [473], [479], a.o.
Special objects include waste rock fill of exploited sections and pillars. Carbonate-rich gangue is easily dissolved,
resulting in calcite and aragonite stalactite decorations inadits. The stalactites are often coloured in blue (Cu), violet to brown pink (Co), rusty (Fe), and most frequently
yellow (U). In the Geschieber vein, calcite drapes 0.75 mlong formed during the 20 years after mining was terminated. Besides calcite stalactites, stalactites of pure znucalite up to 3 cm long formed in the Evangelist vein. Up
to 1 m long stalactites of muddy glockerite were observed at places with rich pyrite. Brown, rope-like accumulations of algae proliferating on rotting wood supports
are impregnated and lapidified by infiltration of calcite.

Massive arsenic aggregates are less susceptible to weathering than complex ores. Arsenic accumulations are locally covered by powdery arsenolite coatings and small
pharmacolite crystals. Rather different behaviour has
been noted with porous arsenic from the Geschieber vein.
In addition to relatively abundant arsenolite and claudetite crystals, aggregates of strongly acidic kaatialaite occur here, with compact crusts of scorodite in their proximity. This type of arsenic contains disseminated pyrite,
which on weathering accelerates the dissolution of arsenic. Some specimens of this type of arsenic, without
scorodite, carry droplets of a strongly acidic liquid, which
persists on these specimens for years, when stored in adry environment. It is assumed that the drops are formedby hygroscopic arsenic acid. Other specimens of porous
arsenic carry prisms filled inside with powdery sulphur
and crust of pyrite or acanthite. These pseudomorphs areclearly after argentopyrite.
In the Geschieber vein, a vertical chimney penetrating lenses of native arsenic was used for draining waterfrom an adit. In areas where water flowed on arsenic,
dendritic silver precipitated. As the place is no more accessible, it is uncertain whether silver precipitated fromflowing mine water containing Ag+ ions or from leaching minor proustite disseminated in arsenic. Silver dendrites were up to 1 cm long, they precipitated within
10 years after this place was mined in 1962 (samples werecollected here in 19721973).

Secondary bismuth minerals do not accompany rich native
bismuth accumulations associated with Ni and Co arsenides; their formation in historical times has not been observed. On the other hand, secondary Bi minerals formed
where bismuth was dispersed as a minor component in theores and particularly where bismuth was partly replacedby bismuthinite. Exceptionally, bismuthinite needles protruding to vugs in porous gangue are coated by a thin layer of bismutite, usually without other mineralization.

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187

Alteration of arsenides, sulpharsenides and sulphides

Alteration of Cu minerals

The 12th level of the Svornost mine exposed strongly
crushed and weathered Geschieber vein without noticeable
mineralization with rich water supply, accompanied byrecent growth of köttigite crystals, almost on day-to-day
basis.
In the proximity of borehole HG-1, near the contactof granite with metamorphic mantle, the discharge of theBìhounek radioactive spring, in places with massive ores,
is accompanied by a rich assemblage of secondary minerals, mainly sulphates. It includes nickelhexahydrite,
morenosite, jarosite, arsenolite, kaatialaite, venekite,crusts of magnesian annabergite, and extremely rare minerals vajdakite [472] and cobaltkoritnigite. Highly acidic minerals such as kaatialaite and venekite, acidic aswell as basic minerals (jarosites, copiapite) occur here
in close proximity. Sulphates of vitriol type co-occur witharsenates krautite, rösslerite, a.o. Cations include nearly
all ore components (Ni, Co, Zn, Fe), as well as cations
derived from carbonate gangue and from wall-rocks: Ca,
Mg, Mn, and Al, and cations brought by the radioactive
thermal spring (Na, K). The ultimate source of molybdenum contained in vajdakite has not been identified, andpossibly comes from the altered younger granite. Surprisingly, the deepest levels in the mine thus abound withminerals of oxidation zone, including native copper. So
far, no secondary minerals of silver and lead have beenencountered. Jarosite formed in the proximity of strongly corroded proustite crystals, but argentojarosite or native silver are absent. It is possible that silver was removed in the form of soluble silver sulphate or arsenate.

Interesting is also the occurrence of copper minerals at
the Daniel level, at the intersection of the Trojická vein
and Geschieber vein. A quartz lens penetrated by carbonate veinlets contains primary pyrite, bornite and tennantite. Copper arsenates lindackerite, geminite, lavendulan, strashimirite and other unknown Cu-arsenate
species [474] formed in the proximity of primary copper minerals. Nearby porous arsenic was accompaniedby common arsenates of Ca and Mg. However, fragments of arsenic, mixed with relics of Cu minerals possibly in the process of mining, yielded copper arsenates(geminite, lindackerite).
The Geister vein between the 3rd and 7th levels (Albrecht) of the Rovnost I mine yielded accumulations of newly formed cementation copper. A chimney between thelevels, functioning as a water channel, exposed coatingsof hydrated iron oxides and arborescent and flaky copper aggregates on all iron and wooden constructions.

Alteration of pyrite and marcasite
Marcasite weathers more readily than pyrite. Marcasite
decomposition, accompanied by the formation of sulphuric acid, acts as a catalyst, as it triggers and accelerates
pyrite decomposition. In the absence of other components, alteration of marcasite and pyrite started by reaction of FeS2 with oxygen from air in the presence of water (even air humidity). Sulphur changed its oxidation
state. Where the alteration proceeded slowly, some powdery sulphur may coat corroded sulphides. In case of
spontaneous reactions, sulphur quantitatively acquires
oxidation state VI. This process is accompanied by anincrease in the volume of sulphide aggregates, fracturing and coating by rusty films of amorphous ferric sulphates. If hydrolysis of these products sets in, and if pHof the solution increases above 4.5, ferric ion is precipitated in the form of complex hydro-sulphates. Sulphuricacid released by this reaction accelerates weathering ofadditional FeS2 and carbonates. Large volumes of FeS2
resulted in Fe2+ and SO42 concentrations exceeding solubility, and skeletal aggregates of vitriols  sulphates
crystallized on the surface of the sulphides. The presenceof gypsum also serves as an indicator of conditions suitable for the crystallization of secondary sulphates.

Alteration of U minerals
As a general rule, the least developed secondary mineralization occurs in the proximity of carbonate veins. Primary minerals including uraninite are nearly unaffected by
the alteration. Minor coatings of Ca and Mg arsenates andlocal coatings of uranium carbonates (liebigite, schröckingerite, zellerite) were formed. Vein carbonates with uraninite found in dumps are usually preserved rather fresh.
In the presence of significant pyrite, the surface of the samples was corroded but secondary mineralization was limited to an amorphous glassy yellow coating in the proximity of uraninite. Massive uraninite in quartz veins oruraninite veins in the country rock is weakly weathered
to zippeite or uranopilite, depending on the amount of pyrite. If arsenide was present in the vein, the most common
weathering product was (meta)zeunerite. In the presenceof very abundant pyrite, uraninite was coated by a thicklimonitic crust and no secondary minerals were formed.
A different situation exists in the wet environment in
the mine. Uraninite accompanied by pyrite is strongly
weathered, coated by zippeite and uranopilite. A different situation occurs with the carbonates. Secondary uranium minerals are sometimes deposited at a distance fromuraninite without obvious reason or relation to any other mineralization. Dusted walls of adits, representing surfaces with efficient evaporation, represent a favourite site
for the crystallization of secondary uranium minerals. A
similar role is played by pieces of rock lying on the floor,
particularly in the proximity of drainage ditches. Wallsof adits may be covered with small aggregates of schröckingerite on areas of several square meters.
Weathering of uraninite is critically dependent on thecharacter of the environment. It is nearly insoluble under reducing conditions or alters to coffinite [150], but
is unstable under oxidation conditions; the efficiency ofalteration is notably increased in the presence of water.

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In dry environment, uraninite is subject to oxidationby oxygen diffusion into interstitial structural sites.
Owing to the presence of other elements than uranium,
an ideal UO2 cubic structure of the fluorite type is notmaterialized. Limits for oxidation occur with the value
x = 0.25, characterized by a shift in unit-cell parameters, while cubic structure is retained UO2.25, i.e., U4O9)
and at x = 0.33, resulting in a distortion of cubic structure to body-centred distorted tetragonal structure UO2.33(U3O7) [471]. These changes may result in weakened
coherency of granular aggregates, formation of microfractures and increase in the reactive surface of thematerial.
Progressive oxidation toward U3O8 (UO2.67) is accompanied by the transformation of tetragonal structure toorthorhombic, which is associated with a significant volume change. However, this transformation takes place
only at temperatures above 150 °C [458].
In the presence of water (or wet air), changes in uranium oxidation do not exceed UO2.33, since dissolution setsin under these slightly oxidative conditions. Oxidation of
uraninite in water-rich environment is characterized bymore efficient kinetics compared to the alteration in air.
Its alteration and dissolution leads to the formation of phases containing U6+ in the form of very stable UO22+.
Factors controlling uraninite destruction include temperature, pH, oxygen fugacity, and concentration of ionssuch as SiO44, HCO3, HSO4, H2AsO4, and Ca2+, admixtures, grain size, presence of other minerals, especiallypyrite, and others.
Interesting information on underground weathering ofore components was revealed in the historic Giftkiesstollen. These old workings were penetrated during the 1950s
by new works in the course of uranium mining (Jáchymovské doly enterprise). The breccia resulting from old
historic mining, comprising fragments of uraninite, pyrite, chalcopyrite and gangue minerals (quartz, dolomite,calcite), was cemented by gypsum, jarosite, plumbojarosite and a number of uranium minerals (oxyhydroxides, silicates, sulphates, carbonates).
Newly formed products resulting from uraninite decomposition show a certain sequence of precipitation.
This was made possible by the fact that large amountsof finely crushed ore fragments remained on the spot after a blast in the course of mining operation. Crushed ore
included abundant uraninite in grains 25 mm in size.
The effective surface of this material was large and contributed to effective weathering. This material was not
drained by water and was exposed to air with certain
moisture. Uraninite in this artificial breccia was grey tobluish grey, with matt powdery to porous surface, freeof secondary uranium minerals, although they were
present in the proximity. Secondary minerals concentrated
mainly in the proximity of gypsum.
The observed succession of crystallization of secondary minerals is as follows:

Journal of the Czech Geological Society 48/34(2003)

uraninite →metaschoepite (UO2)8O2(OH)12(H2O)10,
schoepite (UO2)O2(OH)12(H2O)12 →becquerelite Ca(UO2)6O4(OH)6(H2O)8,compreignacite K2(UO2)6O4(OH)6(H2O)7 →masuyite Pb(UO2)3O3(OH)2(H2O)3,vandendriesscheite Pb1.57 (UO2)10O6(OH)11 (H2O)11 →uranophane Ca(UO2)2(SiO3OH)2(H2O)5,
sklodowskite Mg(UO2)2(SiO3OH)2(H2O)6,
cuprosklodowskite Cu(UO2)2(SiO3OH)2(H2O)6.
The interpreted precipitation of oxyhydroxides before
silicates is supported by the observation of oxyhydroxides in some samples covered by silicates, mainly small
spheres of cuprosklodowskite; a reversed sequence was
not observed. Otherwise, the two groups of minerals tend
to occur separately.
pyrite → melanterite + gypsum:
FeS2 + 3.5O2 + H2O → Fe2+ + 2SO42 + 2H+
typical way of uraninite oxidation:
UO2 + 2H+ + 1.5O2 → UO22+ + H2O
and examples of hydrolysis of UO22+ ions and formationof basic uranium oxyhydroxides:
formation of metaschoepite:
8UO22+ + 16OH + 8H2O → (UO20)8O2(OH)12(H2O)10
formation of schoepite:
8UO22+ + 16OH + 10H2O → (UO2)8O2(OH)12(H2O)12
formation of becquerelite:
6UO22+ + Ca2+ + 14OH + 4H2O → Ca(UO2)6O4(OH)6(H2O)8
formation of compreignacite:
6UO22+ + 2K+ + 14OH + 3H2O → K2(UO2)6O4(OH)6(H2O)7
becquerelite → uranophane:
Ca(UO 2)6O4(OH)6(H2O) 8 + 6SiO 2 + 2Ca 2+ + 4OH +5H2O → 3Ca(UO2)2(SiO)3(OH)2(H2O)5The strongest representation among newly formed
minerals have oxyhydroxides of uranyl, forming spheroidal
glassy aggregates or highly lustrous minute crystals.
Uranium silicates are less abundant, occur in a powdery
form, such as uranophane and sklodowskite, or form
coatings and small radiating spheroidal aggregatesJournal of the Czech Geological Society 48/34(2003)

189(cuprosklodowskite). These minerals constituted cementof the described breccia.
The presence of other uranyl minerals was sporadic.
They include phases which occur largely in the proximity of fragments representing the source of the respectiveions. The carbonates metazellerite Ca(UO2)(CO3)2(H2O)3and zellerite Ca(UO2)(CO3)2(H2O)5 concentrated always
in the proximity of dolomite fragments, metazeunerite
Cu(UO2)2(AsO4)2(H2O)8 near chalcopyrite. Sulphates do
not show such dependence on the source. Uranopilite(UO2)6(SO4)(OH)10(H2O)12 is generally associated with
gypsum. Rabejacite Ca(UO2) 4(SO4)2(OH) 6(H 2O) 6 was
found on earthy coating of larger rock fragments oraround uranyl oxyhydroxides together with gypsum. The
originally present pyrite fragments were probably completely decomposed. Other types of uranium minerals
were not recorded in this particular assemblage, e.g., minerals of the zippeite group, common in other places, orother uranyl carbonates. The respective ore batch was
obviously poor in carbonate gangue, and the relatively
small amount of pyrite was consumed in jarosite- andgypsum-producing reactions. The lead contained in plumbojarosite is almost certainly of radiogenic origin, sinceno primary lead minerals were found.
Similar conditions of hydration-oxidation weatheringof uraninite were recorded in upper levels of the Evangelista vein. The identified secondary minerals include
oxyhydroxides (metaschoepite, becquerelite, compreignacite), silicates (uranophane, sklodowskite) as well ascarbonates (liebigite, schröckingerite). Somewhat morecommon are uranyl-sulphates (uranopilite, zippeite, johannite) and plumbojarosite containing radiogenic lead.
Common jarosite and gypsum also occur.
FLUID INCLUSION STUDY
Fluid inclusions in barren quartz, fluorite and carbonatesof the SnW sulpharsenide, arsenide, arsenicsulphideand sulphide stages of the vein NiCoBiAg±U mineralization were studied at Jáchymov. The results also comprise the data of Beneová (in [359]) on dark violet fluorite of the carbonateuraninite stage.

T a b l e 5 . Results of fluid inclusions measurement from Jáchymov deposit.
samplemineral

FI type(mineralization stages)
Sn–W sulpharsenide stage
J066P

LVR

Th

Tm

salinity

Tfm

Ts

Tm CO2(L/L+V)

(°C)

(°C)

(wt. %)

(°C)

(°C)

(°C)

20
35
2.4
33
7.6–9.5
7.3–7.9–52.5

Trojická veinquartzquartzquartzquartzquartzquartz

prim
prim
prim
prim
prim
prim

0.6
L+V+S
0.2
L+V+S
0.6
0.7

Carbonate–uraninite stage
dark violet fluorite [359]

fluorite

prim

0.9

prim
prim
prim
prim

0.95
variablevariable
0.9

96–112 –25.8 to –29.2
118–142 –22.4 to –25–22.4 to –29.6
118–140 –25.2

23.5–24
22–23
22–24
23

J008I2
J019P
J119P
MP271D

Fe-dolomite
ankerite
dolomitered dolomite
yellow
dolomitecarbonatecarbonatecarbonate

prim
prim
prim
prim

0.9
variable
0.95
variable

126–144 –22.6 to –30–38112. 114 –8.9
–0.2 to –0.5

22–24.5?
12.7
0.4–0.9

Arsenic–sulphide stage
NM9527
J009I2
J009I1

Mg-calcite
Fe-dolomite
dolomite

prim
prim
prim

0.95
0.95
variable

112–123
112–138 –24.5 to –25.7–0.2

23–23.5
0.4

calcitecalcite

sec
prim

0.95
L-only

70–80
< 100

3.6–4.0
0.2–0.5

J107P

Arsenide stage
J005I2
J006I
J007I
J008I1

Sulphide stage
J004I

294–306 –19.6
246–1.4
210–218–4.8 to –6.2
246–262–4.6 to –5.0

264
–56.5
240–246–34

190–198–2.1 to –2.4
–0.1 to –0.3

–55.8
–54

–60

temperature of homogenization (Th), temperature of the first melting (Tfm), temperature of melting of the last crystal of ice (Tm), temperature ofmelting of CO2 (Tm CO2), temperature of solid phase dissolution (Ts). L – liquid phase, V – gaseous phase, S – solid phase, FI type: prim – primary
inclusions, sec – secondary inclusions.

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Methods

Carbonateuraninite stage

A microthermometric study of the fluid inclusions wasundertaken on doubly polished sections 300 mm thickand on cleaved carbonate chips. Fluid inclusions wereanalysed on a Chaixmeca heating-cooling stage [484],
which was calibrated between 100 °C and 400 °C by
Merck chemical standards, the solidification point of distilled water, and phase transitions in natural pure CO2 inclusions. Homogenization and cryometric data show reproducibility of ±0.2 °C at temperatures below 0 °C and±3 °C at temperatures to 400 °C.
Homogenization temperature (Th), temperature of firstmelting (Tfm) and of the final melting of ice (Tm), andtemperature of melting of solid CO2 (Tm CO2) and temperature of dissolution of solid phase (Ts) were measured.
The salinity of the fluids was calculated according to
Bodnar and Vityk [481] and the composition of the salt
systems was determined according to Borisenko [482].
As calcite, dolomite and ankerite are cleavable minerals, fluid inclusions may change during subsequent evolution events and even during the preparation of the wafers and the microthermometric measurements ([480],[483]). Metastability, leakage, stretching and decrepitation of inclusions in these minerals can occur and haveto be recognized and taken into account in the interpretation of the microthermometric data.

Beneová (in [359]) studied fluid inclusions in dark violet fluorite, which was sampled from U-rich mineralization. Primary aqueous fluid inclusions homogenized attemperatures between 190 and 198 °C. Pseudosecondaryand secondary inclusions homogenized in the intervalfrom 105 to 158 °C.

Results
SnW sulpharsenide stage
Primary aqueous inclusions in quartz of the J066P and
J107P samples have a variable liquid-to-vapour ratio(LVR). Vapour-rich inclusions with LVR = 0.2 and liquidvapour inclusions with LVR = 0.6 can be found.
Moreover, L+V+S inclusions with NaCl daughter crystal and vapour bubble of about 30 vol. % were observed.
Very small amount of CO2 was identified in vapour-richinclusions. The temperature of melting of solid CO2 wasmeasured at 56.5 °C, which corresponds to the temperature of the triple point of pure CO2. The salinity of vapourrich inclusions is low, of about 2.5 wt. % NaCl equiv.
Homogenization temperatures of inclusions with LVR= 0.6 were measured between 294 and 306 °C. The salinity of these inclusions varies between 7.6 and 20 wt. %NaCl equiv., the salt system of the solution is formed by
CaCl2, NaCl, KCl, and probably MgCl2 (Tfm = 52.5 °C).
The dissolution of NaCl daughter crystal in L+V+S
inclusions was observed between 240 and 264 °C. The
bulk salinity, estimated using Ts, is from 33 to 35 wt. %.
The relationships of these three fluid inclusion types
are complicated; the preliminary data suggest that the inclusions represent a relatively high temperaturehigh salinity post-magmatic fluid.

Arsenide stage
Fluid inclusions were studied in several carbonates of dolomiteankerite composition (J005I2, J006I, J008I1,
J008I2, J019P, J119P, and 217D). Primary inclusions ofaqueous fluid have variable shape, size of max. 80 µm,
and a variable liquid-to-vapour ratio. L-only, V-only,
V>>L and L+V (LVR = 0.9 to 0.95) inclusions were
found in 3D distribution. Variable LVR is believed to becaused by the long-term maturation of inclusions and nucleation of vapour phase under relatively low temperature [581]. Homogenization temperatures were measured
in groups of the inclusions with LVR = 0.9 to 0.95 andranged from 96 to 144 °C. Tm of ice was observed between 22.4 and 30 °C, and the bulk salinity of inclusions is between 22 and 24.5 wt. % (for H2OCaCl2 salt
system). Tfm was measured in the range from 54 to 
55.8 °C and indicates a CaCl2NaClKCl±MgCl2 salt
system in the solution. Lower salinity was observed in
the J119P sample, of about 13 wt. % NaCl equiv. and especially in the 271D sample, where the salinity of fluidin carbonate adjacent to proustite is very low, from 0.4
to 0.9 wt. % NaCl equiv.
Arsenicsulphide stage
Primary H2O inclusions in Mg-rich calcite of the NM 9527
sample show a consistent LVR = 0.95. The inclusions homogenized at temperatures between 112 and 123 °C. The
shape of vapour bubble was deformed during cooling experiments, and the bubble obtained the original shape attemperatures of about +10 °C. This phenomenon is supposed to be caused by the formation and melting of clathrates of hydrocarbons. Thus, the vapour bubble probably
contains a very small amount of hydrocarbons.
Primary H2O inclusions with variable LVR (L-only,
V>>L, L+V inclusions) were found in Fe-rich dolomite(sample J009I2). Homogenization temperatures were
measured in L+V inclusions with LVR = 0.95 between
112 and 138 °C. The salinity of inclusions is high, of
about 23 wt. %. Tfm = 60 °C and indicates the dominant portion of CaCl2 in the solution.
The later dolomite of the J009I1 sample forms the hemon the older dolomite. Primary H2O inclusions in thisdolomite show a variable LVR and, in contrast, very low
salinity, of about 0.4 wt. % NaCl equiv.

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191

Sulphide stage
Calcite of the J004I sample contains primary H2O inclusions. The inclusions are liquid-only and indicate lowtemperature of trapping of the inclusions, below 100 °C.
The salinity of primary inclusions is very low, from 0.2
to 0.5 wt. % NaCl equiv.
Secondary H2O inclusions along healed microfractures showed a consistent LVR = 0.95 and homogenizedat temperatures between 70 and 80 °C. The salinity of
secondary inclusions is low, from 3.6 to 4 wt. % NaClequiv.
Fig. 76. δ 13C vs. δ18O in carbonates of hydrothermal veins of the
Jáchymov ore district.

Discussion
The character of fluid inclusions in minerals of various
paragenetic stages of the Jáchymov deposit indicates along-term evolution of the hydrothermal system. The first
generation of veins connected with the SnW sulpharsenide mineralization opened during early post-magmaticstage. Carbonateuraninite, arsenide, arsenicsulphide,and sulphide mineralization stages can be recognized during periodical opening of the veins under lower pT conditions. The high salinity of fluids suggests a deep circulation of ore-bearing fluids. The latest vein minerals
formed under temperatures below 100 °C, from low salinity fluids.
Speaking about U-bearing mineralization, the obtained
data can be compared with the results of fluid inclusion
study from the Pøíbram and Roná uranium deposits.
Water-rich fluid inclusions from K3 and K5 calcite of the
Pøíbram U deposit [582] showed Th between 62 and
131 °C and a low salinity, which does not reach 5 wt. %NaCl equiv.
Carbonates K4 of the U-bearing stage from the Ronáuranium deposit [583] have Th from 150 to 180 °C andthe salinity of fluids up to 15 wt. % NaCl equiv.
The introduced temperature intervals of the formationof U mineralization correspond very well to the data of
Poty and Pagel [584]. They suggested that most U-bearing mineralizations throughout the world formed undertemperatures in the interval of 90 to 250 °C.

STABLE ISOTOPES
Nine samples of carbonate [343] were analysed (see Table 6 and Fig. 76). Isotope data of oxygen and carbon
are in most cases homogeneous, which indicates relatively uniform source of carbon and water in the hydrothermal process and also stable physical-chemical conditions
during the formation of carbonates, especially a limited
variation in temperature.
Carbon of hydrothermal solution had calculated isotopic composition of about 6 to 8  PDB. According to
Robinson [486], the supposed temperature of the formation of carbonates is 150 °C. Carbon with such isotopic
composition may be mantle-derived or may represent theso-called homogenized carbon of the Earths crust. Carbon of sample J009I1 originated probably from marbles
which occur around the deposit, and from which the carbon was mobilized during the hydrothermal processes.
Carbonate J009I1 forms white thin rhombohedral crystals in vugs. It is the youngest carbonate on the sample,
i.e., it is younger than carbonate J009I2.
Calculated isotopic composition of oxygen of water inthe hydrothermal solution was about +5 to +6  SMOW
(according ONeil et. al. [588]; supposed temperature of
the formation of carbonates is 150 °C). Such water may

T a b l e 6 . Isotopic composition of O and C () in carbonates of different mineralization stages ofhydrotermal veins in the Jáchymov ore district.
samplept.

mineralδ13C PDB

δ18O PDB

δ18O SMOW

J004I
J005I
J005I
J006I
J007I
J008I
J008I
J009I
J009I

1
1
2
1
1
1
2
1
2

calcite

–6.3
–6.2
–4.0
–7.0
–4.0
–6.1
–6.0
2.1
–5.8

–11.1
–12.6
–10.3
–11.9
–9.5
–9.1
–9.1
–11.7
–12.1

19.4
17.8
20.2
18.6
21.1
21.5
21.5
18.8
18.4

Fe-dolomite
ankerite
dolomite
dolomite
dolomite
dolomiteFe-dolomite

stagesulphidearsenidearsenidearsenidearsenidearsenidearsenidearsenic-sulphidearsenic-sulphide

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Fig. 77. Distribution of δS34 ( CDT) in sulphides from Jáchymov and surroundings according to [342].

be most probably either magmatic or metamorphic water, i.e., water isotopically equilibrated with metamorphic
or igneous rocks at high temperature.
According to the data in [342], δ34 S values range fromca. 21 to +17  CDT  see Fig. 77. The lack of information on mineral species and mineralization stage of therespective samples analysed for S isotopes precludes adetailed interpretation.

Significant fluctuation in δ34 S values indicates changes
in the source of sulphur of hydrothermal solution and/or(more probably) indicates changes in the physical-chemical conditions during the crystallization of sulphides(mainly pH, Eh, oxygen fugacity, sulphur fugacity).
As isotopic equilibrium among sulphides was not attained, the δ34 values cannot be used for thermometric calculations.

Rudotvorné procesy a minerální pragenze jáchymovského rudního okrsku
Práce charakterizuje hydrotermální ilný systém tzv. pìtiprvkové AgNiCo-Bi±U formace. Diskutovány jsou odlinosti geneze U-formace aAgNiCoBiAs formace. Na základì rozdílného fyzikálnì-chemického chování prvkù AgNiCoBiAs je navren model prvkové frakcionace
hlubinných hydrotermálních zdrojù. Je diskutována role arsinu (AsH3) pro mobilitu triády prvku NiCoFe. Je navreno nové schéma mineralizaèních
stádií zohledòující tøi rozdílné mineralizaèní zdroje. Je navrena nová geochemická interpretace hydrotermálních procesù jednotlivých mineralizaèních
stádií vèetnì detailní diskuse termodynamických podmínek mobility UVI a precipitace uraninitu. Dále jsou detailnì diskutovány podmínky krystalizace
Ag a Bi dendritù, precipitace NiCoFe polyarsenidù a elektrochemické rozpoutìní Ag. Jsou poèítány termodynamické charakteristiky pro predikci ainterpretaci geneze vzniku rudních minerálù ve vztahu k mìnícím se Eh a pH podmínkám. V èlánku jsou uvedeny grafické interpretace chemických
analýz, prvkových korelací a trendù pro vybrané minerály. Dále je korelován vznik sekundárních minerálù s pøítomností primárních minerálù. Rovnìjsou studovány závislosti zvìtrávacích procesù v souvislosti s dùlní èinností. Pro interpretaci rudotvorných procesù jsou uvedena data z mìøení fluidních
mikroinkluzí a stabilních izotopù.